For crossover events, it would be nice to see a mix of non-ff properties and other properties from games and none.

For non-ff properties two biggest would be Valkyrie Profile or Secret of Mana.

Though in branching out, one can make the argument of doing a crossover involving Nintendo. Already Square and Nintendo do have some relations given the FF characters in Smash Brothers so I think it wouldn't be a stretch to pull something off.
As to what... well Pokemon would be a solid choice in that there is just so much that could be done with that property. Minions, mounts, and outfits come to mind easily. Though fighting a legendary could be fun. Perhaps a dungeon where it is in the fashion of a gym battle? But given that crossover events are some form of advertising/marketing Legend of Zelda is a contender given the new Breath of the Wild game coming out.

Considering other gaming IP's and now we have the Aglaia alliance raid, makes me want to see something involving God of War.

And I wouldn't also mind seeing non-game IP's making an appearance such as Dungeons and Dragons. Ideally, something involving a quest series from one of the classic adventures such as Curse of Strahd or a newer ones like Storm King's Thunder.