@∫tan x dx: I'll quickly comment on each of those:
The Ballooner: I don't see how this skill would do anything that a Glider in an updraft couldn't accomplish as well?
The Archer: Already ruled out because it would require "assign the skill, assign the direction" in order to be as versatile as needed.
The Rock Climber: Since we've already modified the Climber to have many of the behaviours that are unique to the Rock Climber in L2 (transitioning to Shimmier, holding on to walls after jumping etc.), and because Shimmiers can also cover those 45-degree slopes (just looking in the other direction), I think this skill would have even less to offer now than it would have had in direct comparison to the more limited L2 Climber.
The Kayaker: Of course, in general it would be interesting to have a non-permanent version of each permanent skill (as in the gimmick "non-permanent skills" back in NeoLemmix 1.43 / on the Ephemeral rank of Lemmicks), but since we only have one more skill we can add, I think it would be a waste to use this last slot for something so minor.
The Flamer:
In the joke-skill-suggestion thread that namida linked to in the opening post, I already suggested something similar that would be a little more versatile than just being resistant to fire objects. This skill was called the
Tank (a lemming covered in armour / metal plating) and
would also make the assigned lemming immune to triggered traps and zombies. His weak spot would be water, because he would still drown. And I even said it would be debatable whether it should be possible at all to assign a Swimmer to a Tank, or whether those to skills should be just as mutually exclusive as Floater and Glider. Speaking of Gliders, if the logic is that the Tank can't be a Swimmer because he's too heavy, he probably shouldn't be able to use updrafts either. That begs the question whether it should be impossible to assign Gliders and/or Floaters to the Tank, too, and whether his armour would prevent him from splatting by itself already, or whether splatting would be another weakness of this skill. Maybe he would even splat after falling out of an updraft, because the updraft wouldn't reset his fall height as it does for other lemmings. But those are detail issues I think we could postpone discussing until we actually decide whether we want to pick the Tank as the 20th skill.
The Tank. A permanent skill which makes a lemming invulnerable to fire objects and triggered traps. The animation would be a lemming covered in metal plating. As such, this lemming could still drown in water areas. It would be debatable whether he could still be turned into a Swimmer, as "drowning in water" is just a regular behaviour this lemming would keep, like any other lemming - or whether it should be made explicitly impossible by making Swimmer and Tank mutually exclusive, much like Floater and Glider. In contrast to the Disarmer, it's only this individual lemming that gets immune to traps, which seems to have more uses than disabling the trap for good once a single Disarmer touches it.
Also, the Tank could safely walk among Zombies without getting infected - but wouldn't actually end up killing them. He'd simply pass through them.
All in all, this is similar to what Ghosts used to be, except that Tanks can of course still interact with exits and other objects (pickup skills, buttons, splitters, teleporters etc.). Ghosts would also ignore water, which is still lethal and thus the Achilles's heel of the Tank. Finally, Ghosts encountering zombies would end up intimidating those just like regular lemmings, causing the zombies to turn around, whereas the Tank would just ignore them and walk right through them. He could be assigned a Blocker, though, to turn zombies around.
While being able to walk through fire, traps, and zombies bears some resemblance to Ghosts, which namida has already ruled out, he did mention the Tank as worthy of consideration. In contrast to Ghosts, the Tank can still interact with most other objects, such as pickup skills and exit buttons. You could also make him walk over one-use traps to disable them without having to sacrifice a lemming. The Tank would be less "overkill" than the Disarmer when it comes to traps, because only this individual lemming would be safe from them, rather than disabling triggered traps that aren't one-use traps for good. So the crowd would still have to find a different path than e.g. a Tank pioneer.
Also, the Tank wouldn't display all this less-intuitive behaviour about Ghosts, like not being able to be saved (because the Tank wouldn't ignore exits, in contrast to Ghosts), nor would he have this unintuitive "intimidation" behaviour.
Speaking of unintuitive skills
:
The Rusher: I'm sorry, but the ways you proposed how this skill would affect the execution of every other skill seem quite random to me. Simply performing skills faster is one thing, and I think all that is currently being proposed under the term "Turbo Lemming" here. But this speed coming at the cost of less efficient skill performance, while nice in theory, affects all the other skills in so vastly different ways that it would become pretty much unpredictable to a new player how any given skill would perform if assigned to a Rusher. Even once he has tried out every single other skill (which there would be 19 of at that time!
) in combination with the Rusher, that's still a lot more combinations than he will be able to memorise quickly. (Human working memory can usually only hold 7 +/- 2 different items.)