What about rotated and inverted exits (which this topic was originally about)? Are those going to stay?
I don't see these disappearing any time soon. I've recently even lifted most of the restrictions the editor places on flipping / etc certain object types, though I've left them in place for a few specific types where it could cause technical issues either now or in the future. (Exits are not one of these types; they can be freely inverted and rotated - IIRC it was just entrances, preplaced lemmings and pickup skills that can't be, although the first two can be horizontally flipped.)
It is possible in SuperLemmini. I have made a level with an invisible bridge called Leap of faith. The idea is that the player runs out of builders before getting to this part of the level, but (hopefully) discovers that they are able to walk across anyway!
I would consider that a fair, if somewhat cheesy, use of invisible elements. It's obviously done as a nod to Indiana Jones and is not intended to troll the player, nor does it cause them to unnecessarily die. In fact - it's a help!
I do have another level which is made entirely of invisible elements, but that's the idea of the level and - again - it's done in a very obvious way, not intended to mislead the player at all. It's really a one-off gimmick level, and is done as such. If I was using invisible elements in almost every level without making a thing of it (for example, calling the pack "Invisible Lemmings") then I could understand people being put off.
These are both textbook cases of troll levels. In the first, you are essentially presenting the player with what looks like a gap they can't cross, and expecting them to guess - against standard Lemmings logic - that they will walk across it anyway. In the second, the entire level is up to guesswork rather than strategy - until the user turns on Clear Physics Mode, which they'll likely do right away, and now it's just a standard level except for the Clear Physics Mode graphics. Or if they're using SuperLemmini, which doesn't have Clear Physics Mode, they're more likely to skip the level, or perhaps modify it to reveal the hidden elements.
If these levels are good levels in and of themself, make the invisible parts visible. If they're bad levels and the only reason they exist is the invisible stuff gimmick, get rid of them - the invisible stuff doesn't make them better, it makes them worse.
The golden rule is - a player who's familiar with the graphic set your level is using, should be able to take one look at your level and know exactly what's solid vs not solid, what's steel, what will kill their lemmings, etc. 9 times out of 10, invisible parts violate this rule.
The feeling here generally seems to be that invisible elements are not a good idea though, but I would disagree that if such things weren't included in the original game then perhaps they shouldn't be included now. The original games themselves (including the Oh No! Levels) featured plenty of amusing one-off gimmick levels here and there; Introducing SUPERLEMMING, Lost something? and Flow control spring readily to mind - all examples of ideas being used once for the sake of variety and inclusion, but generally not being applied to the rest of the game. Plus, NeoLemmix seems to be resplendent with new features (buttons, splitters, pickups, more than double the amount of original skills, the ability to assign Lemmings to a particular exit, etc) so why would the occasional well-used invisible element be such an issue?
I disagree with lumping Flow Control in with the other two. "Inroducing SUPERLEMMING" is a relatively easy level in and of itself, that's given fake difficulty from the speedup. "Lost Something?" was tolerable, even fun, in the original game because it was the only time we'd seen this idea - it's an idea that's interesting the first time, but very quickly gets annoying after that.
"Flow Control", on the other hand, operates completely within the usual rules of the game. When solved as intended, it uses a trick that no other level does (this trick can be averted with backroutes, at least on DOS / Amiga where there's a miner-bomber glitch that's very useful here), but the trick can be used elsewhere, and indeed
is used elsewhere in some challenge solutions. The key difference though, is that Flow Control does not mess with how the game works in any way or hide anything from the player, it just requires realising an obscure trick - and
that general idea is not unique to Flow Control, there are a lot of levels that have unique tricks (for example, the trick used at the end of Mayhem 20). While there's probably a few people out there who disagree, "Flow Control" would
generally speaking be considered a good level even by modern standards.
You're not alone in thinking that invisible / hidden elements are fun. Almost everyone does when they first start creating levels - myself included back when I started. And that's where the problem lies - it is indeed an interesting concept when it's done
once. It's just annoying when you encounter it for the 100th time. And most of us here have had those first 99 encounters by now...
In regards to the new elements - the difference is, they're still consistent. Yes, DOS Lemmings didn't have splitters for example, but splitters don't mislead in any way (assuming they haven't been designed with a misleading graphic, of course). If the splitter points left, lemmings go left. If the splitter points right, lemmings go right. It's a new rule, but it's a consistent, predictable one that once a player understands how a splitter works, they can look at any level that contains one and know what will happen.