Tree: 5 remaining slots - a bird trap (birds eating lemmings; you can place that on the nest terrain piece), updraft, one-way fields
I'm only going to add *new* object types, except maybe with the Xmas and Sega styles. Tree style already has a triggered trap anyway (the falling clump of leaves).
Also, are you able to add steel to the Bubble set?
Not at this point in time, as it already has 64 terrain pieces, which is the maximum. I'd have to overhaul the format much more significantly in order to add extra terrain. It's something I might consider eventually, but definitely won't be doing in the next update at least.
You could of course create a customized version of the style that either replaces some of the existing pieces, or alternatively, use the spare object slot (make an object that looks like a steel block, and use the Only On Terrain property - if you want different sizes, you could even designate the object as a pickup skill, with a trigger area in a location that can't actually be reached if the area its on is set as steel; alternatively the Steel object type will be working properly again in the next update so that'd avoid the need to manually set steel areas when using the workaround, though I haven't yet tested how this interacts with negative steel).
Another thing that makes this different from backwards lemmings. For example, after a lemming finishes building, and you assign them a builder again, they will turn around and build in the opposite direction; if you keep doing this, you can make a zigzag staircase without blockers; these will especially be useful as splatforms.
That's actually a really good point. Okay, I'll do this gimmick at some point. It probably won't be the next update though; I need to implement the new extensions to the graphic set formats, then also do an editor update to get it in line with new version features (as well as implement support for new SuperLemmini features), so apart from that and fixing any bugs I might come across, I think I've implemented everything that I'm going to for the next update. (Also keep in mind - you probably saw this coming; since the only function of the new graphic set extensions is to allow more/larger graphics, I'll be updating CustLemmix to support it too.)
At this point, I've finished updating LemSet to support the graphic set extensions, and I've done all the graphic sets except Fire and Crystal (these two, along with Brick, ran into problems before; but since Brick has the simplest arrows, I used it while testing the fixes); I still need to update the player and the editor.
EDIT: Player is now updated to support it. So the player can pretty much be released once I (re-)do the Fire and Crystal sets. An updated editor may have to wait until tomorrow as there's a fair bit to do on it. With that being said, the extended graphic sets don't
crash the existing editor, they just look weird, so you can already make the levels using it if you can tolerate slight graphical issues in the editor.
For those who know the technical details of the formats, the new extensions allow the object section to be up to 4GB (which realistically will probably never happen - the brick style, which is the largest so far (though Fire or Crystal might end up exceeding it, we'll see), is only about 65KB - barely over the limit of the old format). The terrain section's limitation isn't so straightforward - the section itself can be up to 4GB theoretically too, but the furthest into it any terrain piece can start is at the 8MB mark exactly; since the maximum size of an individual piece is 248x255, this puts the practical maximum at about 8.05MB. Still far higher than any graphic set is realistically likely to ever use, especially since the current format only allows for 64 terrain pieces (and if I were to make a new format that allows for more, I could then also allow it to make use of the full 4GB maximum section size - the 8MB-ish limit is due to the limitation of bytes to store the piece graphic addresses).
(And for those of you who don't quite understand the above - this is not the same as the size of the actual source BMP files. The size of the 'sections' in the output graphic set are only about 20% of that of the source files.)
Other new objects (apart from the one-way downs) will come in a later release, since they're a lot more work - I have to actually make new graphics (unless anyone else wants to contribute?) for the most part, rather than just rotate and maybe slightly modify existing ones; although I could possibly recolor some of the graphics used in LPIII to add them to the old styles.
EDIT: On second thought, I will release a V1.20n-A of the editor which supports the new extensions to the graphic sets (but is otherwise identical to V1.18n-D). Then later this week I'll do a V1.20n-B update which adds the support for the new features in NeoLemmix V1.20n and SuperLemmini V0.93a.