Well, I'm not in the plus team so I have nothing to contribute there either. Actually, the best help I can give for plus is STOP TRYING TO SUCCESSFULLY READ .669 files. They are literally just hacked .s3m files, they can be read as if they were .s3m. That would fix the problem of plus breaking .669 compatibility.

Plus on the subject of resolution, I feel like a good first step would be allowing higher resolution in JCS, that way level designers could view maps in 1920x1080 if they really want to, before players open an untested level. Furthermore more memory would have to be allocated to the program to allow generation of objects and rendering of many more sprites and animations. Also, the number of tiles for layer 8 in textured background would need an increase from 8x8 or whatever it is now. A fixed number of tiles might mean a fixed number of pixels. I don't even see a point to that limit other than it being accurate for the official JJ2 tileset skies.