There have been a few crossover topics in the past, and generally it seems people aren’t a fan. Personally I actually tend to agree with this sentiment: not all FE characters are represented to begin with, so crossovers just detract from that. There are a few other reasons, but that’s the one I think I see the most.
With that in mind, I still think there are two crossovers that not only would work, but also be really cool.
The first is pokemon. This has been mentioned before, but it would have the FE characters as trainers, teamed up with a pokemon (in the style of TMS, which they already did a crossover of)
The second is Xenoblade Chronicles 2, with the FE characters as either a blade (e.g. Lucina as Falchion, wielded by one of the XC2 drivers like Rex) or a driver (e.g. a FE character teamed up with a Xenoblade character like Pyra, or one of the other blades). This could be really cool, as the styles of the games match up a little.
Mostly though I just think it would be really funny for Jin/Laura/Hinoka to interact with each other.