I would like someone to write a full-featured battle system interface that anyone could use to add their own battle systems. It is like a plugin system that will maximize compatibility between different battle systems.
The system would allow compatible battle systems to be developed for a single project, provided that they occur in their own scenes (so for example, if you're running a game where battle occurs right on the game map, probably won't be compatible in this system)
However, there are plenty of battle systems that don't take place directly on the game map and so you could just implement your own battle system using the API provided.
Each battle system will need a set of windows, scenes, and possibly their own BattleManager. Each system may need to add additional attributes to other classes like Game Actor, so some standards should be provided to avoid name conflicts if the attributes serve very different purposes (and possibly take different datatypes)
You will preferably need to have experience designing your own battle system so that you have some idea what kinds of things people will need. If you have developed multiple battle systems that might be even better.
The default system should not need to be modified very much.
Scene_Battle and BattleManager will be the public interface that all external clients will access. All battle systems will simply implement a set of methods to actually set up the battle system, and everything else is up to the designer.
Things to think about are stuff like how do you manage windows, since an arbitrary number of windows may be created, and how do you deal with different battle managers.