Wish List for the future
This is the current wish list for SuperLemmix. Whilst the aim is to get all of these ideas implemented sooner or later, some of it might not be possible, and some of it might just not happen, so please don't take any of it as a promise.
I'll aim to keep this list as up to date as possible.

Allow style theme to replace in-game sounds

Assign hotkeys to panel buttons (in addition to individual skills)

Apply recolouring scheme to trap animations

Enable file assocation between .nxrp and SuperLemmix Player

If I can figure out how to get around all the various issues that came up the first time I tried, I'd like to make Blockers assignable-in-water (but with their own dedicated sprite)

Add Horizontal/Vertical Wrap mode as an editor-side design feature

Dynamically resize the skill panel along with the game window - progress on this is slow, but I think I've located most of the code that will need tweaking for this to happen

Make screens Fade In as well as Fade Out, to make between-screen transitions smoother and more Amiga-esque

Give Orig and OhNo styles independent Hi-Res counterpart graphics

Add
Visual SFX (similar to - or the same as - those seen in WinLemm and SuperLemminiToo)

Multiplayer mode. Initially, we'll start with 2-Player and go from there.

Customisable hotkeys in Editor
The following are all discussion-pending, so may not happen:
New objects:

New skills:
Show time taken when test-playing a level, even if the level is failed
Add the option to recolour Swimmer sprites independently of other athletes.
Infinite Skills hotkey.
Once we have Multiplayer Lems, I'm thinking "tribe-specific" Exits might be a good idea. Obviously, in actual Multiplayer, all lems need to be able to access all Exits. But, the concept could be adapted for single-player levels to mean that lems of a certain tribe can only access their own Exit.

If I ever get time, and a significant amount of help, we might be able to
restore .lvl compatibility to the Editor.

Updated steel physics, as follows:

Steel should always be steel, even if it's behind terrain. Currently in discussion
here.

When terrain overlaps steel, allow the
terrain to be destroyed, "revealing" the steel underneath

When steel is
completely obscured by terrain, delete it (to help prevent "hidden steel" trolling)

Allow terrain pieces to be "steel-deleting" in the editor; such pieces will effectively re-shape the steel piece editor-side - this suggestion is currently in discussion
here.

Alternatively, always draw steel to a higher layer than terrain. Then, steel pieces would need to either be pre-decorated, or overlaid with "paint" objects - either way, this would be the best and only way to implement both "steel is always steel"
and "steel is steel wherever you can see it" in the same system!