#31 Alpha / Masked alpha on models. Can make bat wings cheaper this way or torn cloaks. More than one material per model would be nice too for some quake 3 level shaders like glowing eyes or lava flowing down some demons back. All still in quake palette though.
Super8, Engoo and MarkV already supports alphamasked models, and Quakespasm likely does too.
Glowing eyes and flowing lava can be faked by using fullbright colors and animated skingroups. Animated skingroups are part of the original Quake MDL specs but were never used in the main game, so most people aren't aware it exists.
IIRC, support for animated skingroups was broken in GLQuake, but custom hardware-accelerated engines fixed this ages ago.
What I don't remember is if animated skingroups supports custom timing, like animated framegroups does.
Better lava! I want to break up tiling for this stuff. Have it more solid on the shore and flowing in the center. Just have bigger macro textures to break up tiling when seen from a distance. Those large lava lakes could look so much nicer...
Lit liquids with luma textures
can break the tiling. But again, lit liquids aren't going to happen, and I don't know if MarkV and Quakespasm supports luma textures.
I second more advance water volumes that accept lighting and foam.
Foam could be faked by using "negative dirtmapping" on lit liquids. Dirtmapping makes surfaces darker at the edges, but if it suddenly cranked up the lighting at the edges all the way to fully white instead, it could look pretty close to foam.
Even without negative dirtmapping (which could also be called "foam lightmapping") in the light compiler, this could be faked by manually placing lots of tiny lights on the liquids.
The other sensible way would be to implement texture crossfading, which I'm going to do in the future (at the moment I'm faking it through soft depth), but I don't know if any of the other engines supports it. Also, each engine (Q3A, etc.) does texture crossfading in a diferent way, and getting some consensus on standards for such things is near impossible given how different each Quake engine is today.