The sticker idea in and of itself is viable - indeed, I proposed a similar idea but under the name "paint object" as a replacement for only-on-terrain. However, I'd think to be fair, it would need to look completely different from anything that exists as solid terrain, at least in the case where it might otherwise make steel / nonsteel unclear.
If they were to exist as both paint objects and regular terrain, I don't think it would be an issue in any level that's not being intentionally misleading - and designers that want to create misleading levels will always find a way.
Suppose that the "paint objects" you propose already existed, so that you could replace some of the moss with no-effect paint objects (leaving the underlying pixels steel), while some would be ordinary terrain. How would you go about deciding which should be which?
If stickers/paint objects
were to exist, I'd be strongly in favour of them existing as a separate object in their own tab (next to sketches and backgrounds).
Regarding namida's concern about them needing to "not look like existing terrain" - as Dullstar has pointed out, there is
currently nothing stopping a style designer from making an entire style of terrain that looks like steel, steel that look like terrain, background objects that look like terrain, etc (except, of course, the knowledge that such a style is very unlikely to get a positive reception). It might seem counter-intuitive, but maybe offering the potential to be misleading on a plate to designers will actually make it less likely to happen.
The point here is context, which brings me to Proxima's question. The answer would be, it depends...
Back when I was making a proposed conversion, I was given instructions to move the moss away from the edges of the steel. Firstly, there are only a handful of levels in L1 that are affected, and some of these are repeats.
Secondly, on these levels, most of the time the moss is well within the steel area, and it's only when it's dangling from the steel that it becomes ambiguous, so there wouldn't actually be that many decisions to make. Furthermore, if stickers were set to only display "on terrain", then it would become obvious quite quickly which ones need to be stickers and which ones need to be terrain.
Another thought occurs at this point, though:
maybe this could be solved from the steel side. Rather than creating new object types or having potentially misleading graphics, why not just make it so that, in the Editor, steel blocks can be set to override whatever is overlapping them via a simple checkbox that can be applied per-instance? The end result will be essentially the same anyway.
I'm glad this discussion is happening, because it's an important one, and I have made my own views clear. However, ultimately,
I am fully in support of whatever namida thinks is the best way to go with this. It's clear that NeoLemmix exists essentially as another platform
to which the OGs can be ported, and - if they are - then perhaps they do need to be modified to suit the platform, as any other port would.
From this POV, maybe steel should not only
never be decorated, but
maybe terrain should never be allowed to overlap a steel block in NeoLemmix. This seems, in fact, to be the only way to enforce the philosophy (and, incidentally, there are plenty of examples beyond moss decoration for which terrain-on-steel can be an issue in NL):
I can imagine it would be easy enough to hard-code steel to always "ride up" to the highest placement index, and thus always be drawn last.