Okay, so there is definitely some interest in enlarging the pack. That doesn't mean it's guaranteed to happen; for one thing, I have to balance this against quite a few people who've said that the pack is a good size already. I'm pretty sure there aren't any currently included levels that should be removed -- I'm certainly willing to listen if you have suggestions for removals, but since we went through a lengthy selection process to make sure there was firm consensus for which levels made it in, I would have to see a very strong shift of opinion against any particular level before I'd consider removing it.
If we don't remove any levels, then we currently have 160 (32 per rank), and only a few possible additions have been suggested. I'm not wild about the idea of having 33 per rank (especially since the original games usually had round numbers or powers of 2 per rank -- 30 in Lemmings, 20 in ONML, 16 in Holiday); but maybe other people don't feel as strongly about this as I do? We could consider going up to 35 per rank, or even 40, but that would require adding a lot of levels that were considered not worthy of inclusion in the original discussion. Also, large ranks may be a little off-putting to players as the landmark of having a rank completed would be a long way off from the start. (One possibility would be to have 6 ranks of 30, 180 in total, but again, that would require finding 20 new levels.)
So I don't know what the best thing is to do here. Maybe I'll leave it a little while longer to see how things go, and put up a poll if necessary.
My thoughts on the specific levels that have been mentioned:
The ascending pillar scenario: The interesting (and difficult) part of this level is containing the crowd at the beginning, with so little space to work with. (I was stuck on this level for about a year!) I think it could potentially be worth including for this reason, even if the rest of it is just building.
Heaven can wait (we hope): This was a striking level in the original game, because of the tight time limit and 100% save requirement. I think (but can't be sure) that even for a new player, it would be less impressive in NeoLemmix, because the player would be used to framestepping to release the crowd at the correct time (although if Redux is the only pack they have played, there are not that many time-limited levels at all). The tactic of trapping the crowd in a digger pit rather than using blockers is a valuable one to showcase, so I'll check whether we already have any levels like this.
Mary Poppins' Land: This is an interesting case, because Nepster argued for removing it and adding Blizzard 1; I was against that; and with the way bsmith counted up votes, that meant neither level got included, even though it's possible Nepster may have felt that at least one of the two should make it in. I think it's a decent level and a clear example of a useful tactic -- but Old MacDonald Had a Farm feels like a more complex and interesting expansion of the same idea, so that would be a strike against it.
Take a running jump: This feels redundant, even with the proposed fix. There isn't much to it other than building a landing bridge, and we definitely have other levels that show that.
Steel Works: One of my own favourites, but that's partly because of its placement as one of only four levels unlocked from the start, so it was extremely daunting to get a glimpse of the level of complexity the game would eventually work up to, and of course very satisfying when I finally managed to beat it. That wouldn't work at all in NL, because all levels are unlocked, so the player has no particular reason to fixate on the first level of the last rank; and in any case we've chosen It's hero time for that position and I'm happy with that decision.
All that aside -- the ending part of the level is good but neither the beginning nor the middle shows anything we don't have in other levels, so overall I'd rather let this one go than try to fix it into a worthy candidate level, since really we would then be making our own level inspired by the original, and that's beyond the scope of Redux.
Oh No! It's the 4th Dimension! -- This has some interesting features but is spoiled by too much building and a save requirement so low that you can let the lower-left lemmings all die. Again, a good candidate for making a derivative level for another project, but not so much for this one.
The Boiler Room -- I do like this level, especially the daunting "this is completely impossible" first impression it gives. Redundancy with Call in the bomb squad isn't an issue since that level isn't in Redux, although the somewhat similar Don't leave any Lemmings is.
Since I'm neutral-to-negative about most of these, my own inclination is to leave the pack at 160 levels (which would certainly be the simplest option!) but again, I want to take everyone's views into account, so please keep the discussion going -- I'm really glad that even after the huge discussion we've already had, people are still willing to take the time to put their views forward.