This is a well defined topic that seems perfectly appropriate for a tag, rather than being an extremely underused pool. There's currently no way to search for crossovers between multiple different installments in big franchises like Gundam or Precure without trying out each combination one at a time. The only points of debate I can see regarding it's usage as a tag would be the following:
- Art from official in-franchise crossover works (eg. Dissidia Final Fantasy and Fire Emblem Heroes). I personally wouldn't exclude them (a piece of in-franchise crossover art doesn't stop being one because it's official), but I could see some contention against their inclusion. On the other side I'm not sure these sorts of copyrights would flood the tag, or if people looking for this kind of art would mind. Obviously these would only be for art from those works that actually feature a crossover and not just official alternate costumes.
- Copyrights that are in the same setting/continuity/universe but are treated as separate entities on danbooru. This would include combinations like Nier/Drakengard and Apex Legends/Titanfall, as well as bigger messes like Megami Tensei and all of it's offshoots (Persona, Digital Devil Saga, etc) or the MuvLuv/KimiNozo/KimiIta trio (separate works within the same setting/continuity with some recurring characters)
- Cosplay. Simple cosplay like post #5789752 typically doesn't qualify for the regular crossover tag, would we keep that convention for in-franchise crossovers?