If you could have any crossover...

[Shougun]

One of the From Software cross overs could be quite neat (Bloodborne, Demon Souls, Dark Souls).

May also suggest Chrono Trigger, Secret of Mana, Legend of Zelda (specifically if possible Ocarina / Majora's Mask), Legend of the Dragoon, Witcher 3, Diablo (fill the screen with monsters lol), FFIX. FFVII seems like a must when that comes out, or at least a good opportunity. Both an event and also a combo buy, which I'm surprised they haven't moved on that stuff more. Their own launcher and combo purchases like Blizzard does (buy our remake of FF"blah" and get X item in FFXIV, and a special avatar).

For the older non-actively/intended to be marketed games though it makes less marketing sense for them to cross over unless it's their own coinciding with an anniversary purchase or something... Could obviously do cross overs for Zelda or Witcher when the company makes a big new game, but something like Chrono Trigger is old and done and their own IP so.. would want that to line up with a remake/sale/anniversary event or someone at work is crazy passionate to drive the whole event on their free time lol.

Could be fun to see larger / longer last content pieces but that's a bit beyond just a cross over. Like I thought some hunts in the world being more like Monster Hunter could be fun, new recipes and ingredients, new system (old if you've played 1.0 lol) to break off specific body parts, almost everything being dodge able, etc. A-RPG hunts. Maybe some soul relic content for From Software games. If they do Bloodborne though then the dark healer concept I suggested would fit better XD.