For a new mirror system in SP2, consider an automatic mirror system like KSP that reflects what part you place and move.
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For SP1, I like the snappiness of mirroring parts, but it's quite annoying when I need to mirror something internal (Like an engine) and being stopped by the "Cannot mirror part because another part exists here" message. Is there a reason for that?
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I'm just throwing my hat in the ring here

It would be best if left flexible with options, but personally I've devolved into using the tool as a quick keybind to essentially duplicate parts. Although the actual function exists by RMB+drag, I like keybinds and it can be faster.
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I fear that the auto-mirror approach is not best suited for SP's style of modeling (direct modeling). With parametric modeling systems like with modern CAD suites, it is easy to deal with mirroring patterns or solids as it is fully parametrically defined, but this is not really the case for the traditional direct modeling approach as with SP.
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As in SP1 the mirror system seems hugely inconsistent. Sometimes it allows me to freely mirror parts with no limit on how many times I do it, whereas other times it somehow detects a part in the way. I have grown out of using the plane mirroring system, but this is more of a function of the kind of crafts I build and their asymmetrical nature.
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The issue, at its core, seems to always be the connections. When focused on building mechanically interactive or otherwise complex crafts with precise geometries, I find that connections in SP1 become so cumbersome such that I outright bypass the built in system and manually rebuild connections using the connections editor. Again, this is a function of the kind of crafts I build, where the actual geometries are well-defined and the building process consists of mathematically deconstructing complex geometries into the more digestible forms in SimplePlanes.

I want to be able to mirror in other directions
Like up and down mirroring would be cool, or diagonal mirroring would be awesome

There's a mirroring problem with multiple cockpit parts that're on different X axis. Let say I have MAIN cockpit at X=0 and another cockpit at X=1. When I mirrored object at X=1, instead of appearing on X=-1 (using main cockpit as mirror point), the mirrored object appeared at X=0. The mirror point moved at center between cockpits X=0.5. It become more complex with more cockpits added to the build. I hope there will be option to set main cockpit as mirroring point

OK, so I love the current system, BUT, mirroring parts over other parts needs to be allowable, because the amount of times 'There is another part' or whatever the error message is, pops up, it means I have to do it fully by hand.
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This said, for a beginner, or for mirroring like half a plane or whatever, it has its uses, So, I think there should be a Mirror Anyway option for the mirroring, so that beginners can still make sure they aren't causing issues, and I can still mirror rotators that might be touching a little.
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Also, i think this was patched but sometimes along lines of symmetry small pieces e.g. 0.01,0.01,0.01 could just get duplicated infinitely

To be honest, I like the mirroring system as it is...BUT...it would be nice if somehow the mirrored parts' connections worked a little better (don't connect to stuff it's not connected to on the other side) and the whole "can't mirror part 'cause a part already exists on the other side" thing would be removed entirely (if possible).
Other than that, I'll get used to any system :)
P.S If this is already suggested, sorry, I did not read every comment.

@CrazyCatZe Yep, I had the same happen when I had parts of a radius =<0.1 in the middle of the aircraft.

As for my own feedback, I'd like mirroring to reflect all the connections I make on one side of the plane, regardless of if proximity is maintained or not. Wing pylons were super iffy when wing thickness was introduced, and it was a massive pain in the rear. I'd also like us to be able to choose if part settings get mirrored or not (maybe a checkbox in the part UI?) as well as the previous mirroring options in SP. Overall that system was most of the way to being great but just needs some touching up to be really great!

As an aside, I think the attachment point UI could scale a bit with part size to make attaching small parts to one another easier, as previously it was next to impossible to attach small pieces in the drag and drop way.

4 way symmetry would be amazing aswll as not messing either non pitch inputs for wings I'm sick of flipping my VTOL flaps it's so annoying

I think a system like Juno that’s a little more flexible, like being able to symmetrise parts that are connected to already symmetrical parts, or where it makes sense to allow two points of connection. Could possibly really help with making things like missiles as you don’t have to worry about breaking their mirroring after being placed on a mirrored pylon under a wing or whatever, probably would end up being a bit hellish to implement but it’d be really neat

The other thing I'd like to add is drag points... let's say I have an issue with drag on one side you'd think mirroring the craft would fix it... the reality check is it doesn't which I find odd

@ZeroWithSlashedO yeah that's the most annoying thing take for example @NathanMikeska if im working on a wing adding pylons with whatever and the wing has different thickness when you mirror a connected part it won't actually connect and so you have to do the other half of the work of the mirror function and suffer to a function that works and functions but not 100%

I'm not sure if anyone else has experienced this issue, since I’ve never heard any discussions about it. It seems like the mirroring system sometimes mirrors parts that are in the middle of the build, resulting in two identical parts in the same coordinates. These duplicates then continue to get mirrored in the same way each time.
I remember working on a small plane years ago and using the mirror function constantly. Eventually, my game started running slowly, and when I checked the part count, it was at some ridiculous number. It turned out a small part in the center of the plane had been duplicated over and over, exponentially increasing the part count. This happened years ago on the mobile version, so I’m not sure if it's still an issue on PC (I haven’t used the mirror tool since switching to PC), but I might have an idea of what causes it.
Sometimes, parts get a strange offset. For example, I build my planes aligned with the centerline X = 0, but after some time, I notice that certain parts have shifted by one unit to the side. I think that because these parts are no longer aligned with the cockpit, the mirroring system still duplicates them but doesn’t remove the original part for some reason.

Personally, my only issue, even if its a major one is with mirrored parts losing their connection points. Or worse of all, connecting themselves to random parts. I feel that in SP2, I should be able to mirror something like a piston with any number of parts attached and expect the mirrored parts to maintain their connections to the mirrored piston and not add any new points.

One issue I particularly have over mirroring is connections. When a part is mirrored, it sets it's connections to Auto-Reconnect. But oddly enough, if you just attach the part and nudge it away then mirror it, it stays connected to the same part.

It feels incredibly inconsistent, and completely frustrating.

I just want to de-select some parts from mirroring. Like the steering wheel on a car, or door layout on a plane.

I would love it if we could select our own preferences (relating to the second paragraph) from inside the builder

I think it would be nice to have both automatic mirroring and manual mirroring if that makes any sense.
It'd be nice to have what you are making, for example a wing, be mirrored in real time as you edit it but then be able to disable the automatic mirroring, to work on something else you maybe dont want mirrored onto the other side of the vehicle, or to edit something on the wing you dont want on the mirrored wing

@PlaneFlightX

This is a relic of many years ago

No single line so succinctly sums up the SimplePlanes experience as this.
From a hotwheels car to a battleship, at it's core it's still a toy.

@V "Cannot mirror because-"
Shut up SHUT UP!
Mirror, damnit!

I am a mirror on control player. I think that any asymmetric parts won't do well with auto mirroring.
I managed to control part-section mirroring by making sure that each part attachment point is only used once. Using an attachment multiple times makes things not mirror friendly.
I would like to see single part mirroring or duplicating on mobile.
I would like to remove the mirror entire plane buttons, since they seem to break the plane even if the mirror isn't carried out.

The one SP1 has is already good though only having a few attachment issues where, when mirroring, connections on one side of the build do not match the other, but it's not really that of a bother since it could be mostly fixed with minor readjustments

However, there is a need for an ability to segment off parts from mirroring, esp when an asymmetrical build is involved.

This game had some pretty detailed mirroring https://www.youtube.com/watch?v=CKvbSz4ZfQY I was (and still am) a huge fan of it

One thing I they did that I would really like to see for part placement is a crosshair mode (center of the screen crosshair for placing things). That way you on mobile, or pc, or even controller, you could have a stable way of placing parts.

An input which disables mirroring for certain parts would be cool

Real time mirroring symmetry would be nice, similar to J:NO. Basically you have an option to radially mirror it or symmetrically mirror, or manually mirror the part you have. Non-radial mirroring should not be relative to a specific part e.g fuselage, it should be relative to a symmetry plane set up by player, you should also be able to move the symmetry plane, and have it in either X, Y and Z axis as well.

The attachment points should be persistent on the other side of the symmetry plane as well, like ONLY attaching to parts that are connected to the original part. Currently it is annoying to deal with the mirror part feature in the game as its attachment points get attached to random, unwanted parts. I have a few workarounds to counter these issues but it takes time atm.

I just remembered one critical problem with the existing system: "Failed to mirror part because another part already exists"
This is a relic of many years ago, and just flat out doesn't work. I was working on my autopilot panel and wanted to mirror a button to the other side (inside the cockpit cavity, there was nothing there), and it wouldn't let me. I suggest either removing this check entirely or adding an option to disable it.