Weasels. Their main function is already covered by Zombies - however, Weasels are somewhat less dangerous, since they don't infect Lemmings they encounter, which, in case of Zombies, can then go on to infect other Lemmings, i.e. the effect is cummulative. Weasels are really more comparable to moving traps, like the bird or the "Lemme fatale" in Lemmings 3.
Indeed, weasels are more or less non-infectious zombies. If we were happy to make them exactly non-infectious zombies, except for visual differences and maybe a small pause (for an "eating" animation), then the biggest issues would be (a) do we need yet ANOTHER type of lemming?, and (b) who's going to make the graphics?
Beyond that, I see little technical difficulty with implementing them, though I'd question how many use cases there are that can't with some slight adjustment be made to work with either zombies or a non-moving trap. Until such cases can be shown, I definitely see no reason to bother with all this.
Conclusion: Extremely unlikely, but not ruled out.
Moving traps. The only one I remember is the spike roll that would circle around the level, killing any Lemming that happened to be at the wrong place at the wrong time.
These in turn would just be weasels, but slightly more different from lemmings - though also likely much easier on the graphical side of things. Most of the same arguments apply outside of graphical ones.
Conclusion: Extremely unlikely, but not ruled out.
Laser gates and levers. These act basically like exit-opening buttons in NeoLemmix - but they work on a more general level and can be used to "lock" or "unlock" other things. For example, filling up a gap with water, or draining the water from a gap, or opening and closing metal gates to certain areas.
This is an idea I've actually considered personally (though not brought to any discussion so far) in the past. I'm open to considering this one for sure.
Conclusion: Potential candidate for future consideration.
Filling up or draining water would sound reminiscent of the rising water gimmick, but in Lemmings Revolution, it's an all-or-nothing thing: Either there is water in a basin, or there is none. I'd very much love to see this feature - it would allow to make levels about creating Swimmer-friendly paths, much like some levels are about creating paths that only work for Climbers or Floaters - but then, filling water into an area should be instant, not a slowly-rising thing. Essentially, working with active and non-active water trigger areas.
This is going to look kind of awkward graphically if it's instant, unless gameplay pauses to show it filling up then resumes, which could get annoying if the switch is repeatedly hit for unreleated reasons. Maybe there's some solution. I definitely do agree that there could be some interesting potential in this one, though the key word is "could", and I think a use-cases trial run (rather than just a bug-testing trial run) would be needed before such a feature would make it into a stable build.
Conclusion: Neutral.
Gates. In general, these are much more versatile than locked or limited-number exits: They can act as one-way terrain (if a Lemming has to get to the other side to push a lever and make the metal pieces retract), they can serve as bridges to cross gaps or barriers to block lemmings, cut off the path to certain pickup skills, only allow a certain number of Lemmings to pass before closing for good, etc.. They might even be able to kill Lemmings (or Zombies / Weasels) if they extend while a Lemming as passing by? In NeoLemmix, such a Lemming would probably only get trapped inside the terrain (although, if that terrain is also considered steel, he would have no chance of every escaping again).
Essentially, this would mean "active" and "non-active" terrain in NeoLemmix, but clearly identifiable visually. A fair version of the Clock gimmick, so to say (though I've never actually seen this gimmick in action), which, according to my knowledge, made terrain present or absent on a mere timing basis (=execution difficulty). With metal gates like the ones in Lemmings Revolution, we could incorporate this idea into puzzles in a fair and predictable way!
I'm a bit wary of the idea of modifying the terrain based on anything other than skill use, though not to the extent that it's a "this is never happening no matter what" thing. There'd need to be some very solid support for it and some good puzzle ideas during a test period though.
I believe you've overlooked one case where a lemming trapped in steel under such circumstances could escape: If the gate is opened again.
The clock gimmick related to real-world time - if you started playing the level at a certain time, terrain would be there, otherwise, it wouldn't. I used this in one level, "The Midnight Bridge", which had a start and an exit seperated by a gap that you didn't have enough skills to cross. If you played the level close to midnight (for this, it just used whatever time your PC was set to; no fancy internet checks or anything, so you could also beat it by changing your system clock), there'd be terrain across the gap, and lemmings would walk straight to the exit without having to do anything. I might be wrong, but I believe this level was the only one full stop that ever used this gimmick.
Conclusion: Unlikely, but not ruled out.
Speed-up pads. These would essentially be to the Lix Runner skill what radiation and slowfreeze were for Bombers and Stoners. They might not be hated as much by the community as a whole, because they don't include a countdown. But they would essentially introduce the Runner skill (or Superlemming gimmick) in specific locations of a level. I actually opposed the introduction of the Runner skill into NeoLemmix in the early days of New Formats, arguing that it would mainly be exploited to create scenarios of difficult execution. Some people pointed out the possible interactions between Jumper and Runner. I still think a skill isn't really viable if it's only worthwhile in combination with specific other skills (like, in this case, increasing the Jumper's distance). But the closer we get to the introduction of the Jumper into NeoLemmix, the more tastes may vary.
Agree; these aren't very useful. If we were ever to get a Runner skill, this could be reconsidered, essentially as an object that gives a lemming the Runner skill. In such a case, I'd probably want to generalise it to objects that give lemmings a permanent skill of the level creator's choice. I'd have to be convinced that there's puzzle potential in this that can't already be achieved, or currently would be too backroute-prone, with pickup skills or teleporters, though.
Conclusion: Ruled out for now. The generalised variant of "an object that puts a permanent skill on lemmings" is unlikely but not conclusively ruled out.
Anti-gravity pads. I've stated repeatedly that I'm a big fan of these, because they add all the versatility of L2 skills like the Magno Booter, or other conceivable skills, such as upward digging or downward building, without requiring the introduction of actual new skills. Then again, the reason they do this is because they essentially work like a gimmick - not like the "no gravity" gimmick, of course, but more like an actual application of "Upside-down world". So while I do believe these have huge puzzle potential, I also acknowledge they'd probably be a nightmare to code, especially whenever any new skills get introduced.
If we ever get reverse gravity, we will have gravity changing pads. Reverse gravity is on the list of features I'd like to have but would also be a huge difficulty to implement - it's very possibly something we'll get someday when I get in one of those major "let's get something HUGE implemented" moods, although I've got a couple of huge features I already need to attend to first.
Conclusion: We'll probably get this someday, but probably not anytime soon.
Teleporters / Portals. The big thing about teleporters in Lemmings Revolution is that they're not release-rate sensitive: They work just like splitters in NeoLemmix and affect every lemming passing through. That's why I'd call these "portals", to distinguish them from teleporters: Rather than triggering, they just change the position of a lemming. I think introducing these, while it might seem redundant to teleporters at first glance, would accomplish several things:
This is another idea which I've given consideration to, and I think even briefly brought up in chats a few times, though never had serious discussion around.
Conclusion: This is a potential candidate.
Break-away floors. I don't actually remember these from playing Lemmings Revolution myself, but these would essentially work like limited-number exits - except that they're limited-number terrain. I wouldn't recommend the terrain falling apart piece by piece, like the rising water gimmick in old NeoLemmix; instead, they could be labeled with a number, just like limited-number exits, and then the terrain would simply disappear once that number reaches zero. Essentially, this could be combined with the idea of gates explained above.
Implementation could get quite messy here, and similar to gates, it's terrain that varies based on conditions other than "a skill has been used".
Conclusion: Unlikely, but not conclusively ruled out.
Some of these would probably be rather easy to introduce, especially things like Portals (=non-timing-based teleporters), which could basically rely on the code for something that already exists, and merely slightly adapt it, while still providing many new opportunities for level designers.
Relying on existing teleporter code for portals would be overkill if the warp is instant (much simpler to just immediately change the lemming's position upon meeting the trigger area), and not useful if there's an animation (think similar to L3D teleporters, except that multiple lemmings can use it at once) due to the way it's written. Regardless though, portals would still be a relatively easy one to implement.