I think it'd be cool... I keep thinking of Slave Knight Gael come to Eorzea. Wonder if he'd be drawn to our soul too?
As for aesthetics...I'd love Artorias' armor, Soul of Cinder and hmm Gwyndolin. I can probably keep going... I think a story line where we're piecing together some lore and maybe progressing abit would be interesting though I have little faith in its execution without it being 'too on the nose'.

Also wasn't there a running joke that I think not sure which tome gear in ShB felt bloodborne/DS inspired? One of the scythes for reaper screams Gehrman.

As for fights, I'm not sure. I personally didn't play DS myself and my interest in it has been lore exclusive, but I was one of the people intimidated by the difficulty but ended up falling in love with Bloodborne, I platinum'd it as well. Was really fun with literally one exception (watchdog of the old lords in chalice dungeon on a uh bloodtinge/chikage, was pure skill issue and I didn't want to deal with him anymore after getting him to 15%).

But doesn't a collab mean we have to send someone over from our side?