My thoughts on all the proposals so far, except the "ruled out" ones. For those who don't want to read a wall of text, I'll use talisman icons:
Yes please!
This has definite potential, and I'd be in favour so long as it doesn't result in something I like
more missing out.
Not sure about this one, and considering we don't want a glut of new object types, that really means I'd probably vote against.
No. This should be eaten by the clam.
* * * *
No-assignment field: Extremely handy as a backroute preventer, but can also be the starting point for a puzzle in its own right (e.g. here's a wall you need to get up; there's a NAF in front of it so you can't just build up).
Ice / slippery terrain: There is a big problem with this idea. We already have ice in some tilesets, and it is
not slippery. Introducing new ice that is slippery when old ice isn't would be very confusing.
Portals: This is what teleporters should always have been, and if we can't just change teleporters to work as portals because of existing content, then adding them as a new object would still be a welcome move. I think it adds enough puzzle potential and (more importantly?) subtracts enough user annoyance to be viable even though it's a variant on an existing object.
Trampoline/spring: Back when Lix had these, I was always against them, but then, Lix trampolines had a variable result depending on where the lix jumped or fell from, so it was often hard to know exactly where you could reach and where you couldn't. As I understand, the proposed NL object is a spring that, essentially, gives lemmings the Jumper skill without consuming a skill. While that does remove much of the annoyance of the Lix trampoline, it's still a niche object that will only see occasional use, especially if we do add portals.
Zombifier / neutraliser and their converses. For starters, I don't agree that we have to have all four if we have one. (If we have more than one, new users have to learn what they look like
and how to distinguish them.) So let's discuss them separately. Lemming -> zombie is the least useful because that's what a zombie already does. Use a preassigned blocker if you want him to be stationary. Lemming -> neutral has some potential if it's unavoidable and you have to babysit the neutral, but does this really add anything we can't already do with neutral preplaced lemmings and hatches?
Neutral -> normal lemming has some potential, because you have to direct the neutral to the object; but "direct the neutral" puzzles can already be done by having to direct them to an exit. Zombie -> normal is the most interesting, because you have to direct the zombie while avoiding being infected. However, this too can already be done, either by having to direct the zombie to its death, or direct a zombie disarmer to a trap.
Permanent skill remover: I can imagine a lot of different puzzles using this, so it definitely has potential. I just don't feel I'd especially feel the lack of it if it didn't get added, though.
No-effect animations: Why? This is just lying to the player, something we've agreed again and again is not NL's philosophy. If something visible happens in response to a trigger, it should have an effect.
Permanent skill assigner: No, this is getting out of hand. Just use a pick-up skill if you want a permanent skill to be only available after reaching a certain point.
Buttons and gates: This has a lot of potential. How I see it working: a gate is an indestructable rectangular object (i.e. all interactions with it are the same as interactions with steel), and buttons can be assigned to open or close certain gates when stepped on. The gate would have three parts and only the middle part would open, so the "frame" would show the presence of a gate, and it would be possible to have gates that start open.
Buttons that can affect any object: This is overcomplicated and doesn't really do much you can't already do with buttons and gates. (I know I suggested it in another topic, but that wasn't intended seriously.)
Gates that only let one lemming past: Not worth being a separate object since this is easy to set up with a button and gate.
Blue/green hatches and exits: To spell out what DireKrow is suggesting, the idea is that there are two sets of lemmings, with no difference between them except that some exits can only take one of the two sets. Of course, they shouldn't actually be blue and green, since we already have green to show that a lemming has a permanent skill. I can see some possibilities for this, but it's already possible to set up, with permanent-skill hatches and two (or more) exits that require different permanent skills to reach.
Permanent-skill-only exits as an object: Redundant. You can already do this with a normal exit.
Up and down one-way fields: There are some situations where these would be handy, but I'm not wild about them.
Diagonal one-way arrows: We have enough one-way arrows already, and the player has to remember different rules for each type. Let's not.
Screen wrap: We have this in Lix, and it's mainly a multiplayer feature, with occasional use in single-player puzzles. I don't think it adds enough to be worth considering at this stage, especially since it can be simulated with portals.
Karoshi: Sure, it was fun as a one-off, but the decision that was made when we scrapped gimmicks is that
the rules of play should not vary from level to level. That's why zombies were promoted to a main feature instead of a gimmick: they don't violate that rule. Namida has made clear that screen wrap will
only be coming back if it's also in a non-violating way: allowing the player to scroll continuously instead of surprising them when a lemming walks off-screen and comes back on the other side.