I think a more fruitful way of going forward would be JJ2+; higher resolutions will almost certainly be possible somewhere down the road. Even now you could create a custom J2A in which sprites would be bigger; of course, this would probably lead to problems with existing levels, but it looks like those aren't in the equation here anyway. As far as I know additional layers are technically not impossible either. The only thing that might be hard to change is the 256 color limit, but I wouldn't put it past the JJ2+ team to somehow work around that too, in the future.