One idea I had thought about, but didn't include, for Lemmings Plus Omega, was indeed secret levels based on a different concept; specifically based on completing certain challenges (to use the original game as an example, 100% on Tricky 25 "Cascade" could be an example of something that might unlock one). This idea could possibly be used for the community pack, although IMO if I was going to do this, I'd have to implement this as a standard NeoLemmix feature that anyone can use in their packs (which to be fair, I don't have a problem with doing beyond that it might be tricky to come up with an exact method of handling it).

Indeed, some of these gimmicks are little more than novelty things without much use for a real level; a lot of the weird ones involving skill counts were done back when I was accepting virtually any gimmick request, while the "reverse skill counts" was originally intended for LPII but I ended up creating the Overflow gimmick and using that instead; there's no real reason why RSC was left in the code, but it was, so it survives still. The "Invincible" one, specifically, wasn't really intended for use in actual levels, but more for testing purposes with various things. Though in comparison to the combo Nepster mentions, there would be slight timing differences due to them falling faster than floaters, as well as that it's possible to assign them Gliders.

I do agree though on the "don't use it just because it's there" option. Sure, if a good way to use it can be found, then go ahead, but if it can't, then I don't see the point in using it just for the sake of using it... At the same time, the "good way" of using it doesn't nessecerially have to be relating to a puzzle. It could just as easily be related to the theme of a level, especially in easier levels.

So, here's some things I'd like to start thinking about; anyone can feel free to give their input:

1. Difficulty ranks. How many should we go for? I'm thinking 4 or 5 is probably ideal, though 3 might work.
2. Gimmick / Frenzy levels. As these are one of the most noticable NeoLemmix features, I think it'd be a shame not to include some. The bigger question is how - LPII-style where they're just mixed in (at a low frequency) with regular levels? LPIII style where they're the secret levels? LPO-style where there's a dedicated extra rank for them?
3. Repeats, or no-repeats? Having them would reduce the number of unique levels we need; we could just fill in early spaces with 20-of-everything versions of harder levels (and there's also the option of cases like with Danger 27 / Psycho 12 from LPI, where the two levels use the same map but have very different solutions).
4. Secret levels or not? Assuming we don't go with the LPIII-style for gimmick levels, then should we still have secret levels? On one hand, they're a pretty unique NeoLemmix feature, but on the other hand, while they were novel at first, I kinda feel they're more annoying that anything else now (hence the complete lack of them in Omega).

There's four days left to get your entries in. So far, I've received three valid entries.

Okay, watched your solutions. Some comments:

Outbreak 3: That has to be the tidiest "100%" solution I've seen yet.
Outbreak 4: That's also a new way of solving it, and the first right-side solution I've seen (though of course I knew it was possible ).

Plauge 1: That's the first time I've seen anyone else pull off "100%" on this level, and like with Outbreak 3, your approach is far tidier than mine.
Plague 5: This is a third way of executing the solution to this level. Nice! It's still very close to the intended solution so I wouldn't call it a backroute; just an alternative.

Apocalypse 1: Your replay doesn't seem to work. It just assigns one lemming a climber who walks into the trap, then nothing else happens. (Maybe it's from the pre-update version? The new version has one less zombie, which somewhat messes up existing replays.)
Apocalypse 2: Congrats on a new saved lemmings record!
Apocalypse 3: Not the intended solution, but I like that one so if I can fix Akseli's backroute without breaking yours (which should be doable), I'm leaving it in.
Apocalypse 4: Backroute; it's similar to Akseli's one. It's gonna get patched.
Apocalypse 5: Backroute; undecided as to whether I'll fix this one.

I'm not planning to add any more skills. There's a topic in the General Discussion board about possible new skills; you might want to post these there, in case someone working on another clone sees them there and thinks "hey, that's actually a pretty neat idea". Thanks for the suggestions though.

That's awesome! The lemmings you drew look really cool too!

I think the problem with levels like that is that they're an amusing twist the first time you play them, but offer virtually no replay value whatsoever - only one solution, and it's one that you'll very easily remember.

Not sure about the traditional Lemmix editor, but in NeoLemmix Editor (which supports regular Lemmini as well as SuperLemmini), you can have unlimited* pieces if you're working in Lemmini or SuperLemmini styles. I'm not entirely sure if Lemmini itself has a limit though; you may want to ask Tsyu (creator of SuperLemmini) or someone more familiar with it about that.

* Obviously, there's the limit of how much memory your PC has, and in practice, there is a limit - but it's one you're not likely to ever reach even ignoring memory limitations; just under 2.2 billion of each type of object.