Rather than the SMTxFE that we got I'd like the FExSMT that I'd always imagined; a modern post-apocalyptic setting with Fire Emblem characters, and gameplay somewhere between FE and Devil Survivor where Fire Emblem units are paired with demons that provide expanded options and different attributes.
100% this, TMS isn't a bad game, i think it's a fine RPG, but i think it struggles a lot with capturing the spirit of both SMT and Fire Emblem as franchises, TMS plays very differently from those franchises, and is very thematically different, that i think it fails as a crossover more than anything else, as a new game in a new IP, i would be fine with it, but as a massive fan of both FE and SMT, i was extremely disappointed by TMS, even though i don't hate it (or think it's bad), just because as a crossover, it failed hard on my book.
Anyway, another Pokémon Conquest or a Pokémon SRPGs made by Inteligent Systems is an easy sell to me, i loved Conquest dearly (and that game itself was also a crossover with another series, the series that was localized in the west as Nobunaga's Ambition, though since it's a really small IP in the west, the crossover branding was removed for the western release) and i love SRPGs in general, and Pokémon worked really well in that context.
A Fire Emblem X Soul Calibur would go so hard, it's the stuff dreams are made of too.
An F-Zero game made by RGG Studio would be pretty great too, even if it's not a racing game, i think the IP would work really well even for a Brawler, and RGG isn't strange to making Brawlers with other IPs (ie: Hokuto no Ken).
I would also really like a Project X Zone 3 with Square Enix added in the mix, for the wacky crossover shennigans, Bandai Namco x Nintendo x Capcom x SEGA x Square Enix would open the potential for so much silliness and fun character interactions.