there might be lots of replay files ... And the levels, too
This is currently not a problem
at all, since no SLX levels exist other than the few test levels I've released just so that people have something to play with.
With that said, it's actually very unlikely that we'll end up swapping the skills anyway, since it would be way more work than it's probably worth. I'm just putting the option on the table in case anyone thinks it's a good way forward.
Let's proceed on the basis that the Twister will be our vertically-upwards-oriented destructive skill, then. I'm thinking strictly vertical (let's keep it as straightforward as possible), and it should be a sudden, explosive action, as quick as a Laserer. Maybe we give the lem a "propeller hat" which shoots them upwards, destroying any terrain they encounter in the process.
I then envisage 2 options for stopping the action,
ruling out limited range or limited time (neither of which are desirable):
When they no longer have any terrain above them (a 5px check, half the height of the lem, seems appropriate, but it could be anything), they complete a "jumper"-style arc in the direction they were originally facing (during which no further destruction takes place), and then fall. Like this:

Or (and this is probably what I'd prefer), the same overhead check for "no more pixels to destroy", but the lem falls back down the newly-created tunnel. Since they're wearing a "propeller hat", this serves as a "Floater" which allows the lem to descend safely. Like this:

Reasons to prefer the second of these two options:
- Far simpler to code
- Less to worry about regarding the "after-arc" in terms of other skill assignments, level design, predictability of movement, skill shadow, etc.
- If the lem is also a Glider, the Twister propeller takes precedent, since the descent will be a predictable and necessary part of the skill action itself
- The action is simply a way to create a vertical tunnel, and nothing else - in this way, it's akin to the Laserer's overall action
We can of course consider how a Walker may interact with this skill. If the Walker is a universal "canceller", then maybe the lem transitions to faller, allowing other skills (e.g. Glider) to be assigned and/or take effect.
Also, I think
"Propeller" is a more suitable title for this skill as it's currently proposed.
Thoughts?