Editor: Those are all features I can consider eventually. For now, I'm just keeping it in line with new NeoLemmix / SuperLemmini features (well, there probably won't be any more new NeoLemmix features now (apart from maybe the full-color graphic sets), but SuperLemmini there may be) and fixing bugs. Once NeoLemmix is complete, I might look at the more flashy stuff.
Extra: I'll look into both of these.
Music packs:LPII has the same musics as ONML, and as such you can simply use that one (the NeoLemmix version player supports it; just rename the files). The gimmick music doesn't sound very impressive in its original state, while the Frenzy music I'm not even entirely sure what the ultimate original source was.
LPIII, I might look into it eventually, though a lot of the tracks are remixes that only exist as midis (however, some tracks such as the gimmick musics and
the first track could be potential candidates; and it *is* possible to mix formats in a music pack). Another issue is that there'd be quite a mix of quality here - the first track is CD Audio, the second, sixth and final-level-of-rank are MIDI remixes, the third I'm not sure what system exactly (SNES or N64 I think?), the fourth, fifth, frenzy and final-level-of-Fierce are Gameboy, and the two gimmick mucis (not counting the LPII one) are PS1. Basing them off MIDI versions, they don't sound too out of place together; if the original tracks were used, it would sound very strange indeed.
HLP, I'm not sure there's much point, as the source musics are midis anyway so the ones in the release pretty much sound exactly like them.
I can say though that there probably won't ever be a non-NeoLemmix version of LPII that supports music packs or in any other way uses the new players - unlike LPDOS, which has exactly one modification (not including the different rank names, texts and level order) from the standard mechanics (the instant bombers), LPII has the gimmicks, secret levels, etc, which aren't supported in the traditional versions of Lemmix. (In fact - not sure if I've mentioned this before - the NeoLemmix project was initially intended to be a player for LPII, but since it was nowhere NEAR ready by the time LPII was, I didn't go with this idea, and instead it wasn't until LPIII that NeoLemmix was being used.)
Testing: Most likely, it'll simply be a matter of "finalised once all features I want to implement are, and it's been out for a while without any issues that need fixing being reported". This won't happen before LPZ is released, as that'll probably be the first chance people have to really test out the new skills - HLP and the tutorial pack give somewhat of a taster, but LPZ will be the first (and from me, most likley the only) full-length pack using them.