Since the soulslike community is quite diverse, I can imagine many different opinions to emerge here. Curious to hear your thoughts!
I'd take Sekiro 2 any day of the week. Bloodborne 2 would be amazing, but Sekiro, to me, is the pinnacle of the genre and has so much untapped potential that's laying dormant. Bloodborne is too similar to your "standard" soulslike compared to Sekiro, even though I love Bloodborne as well.
Sound analysis of why Bloodborne 2 would work well. Concerning Sekiro 2, the true ending definitely leaves the narrative open as to what happens next.
Tap for spoiler
Kuro has been cradled by the Divine Child of Rejuvenation so they and Wolf can bring back the Dragon of West to the West - likely China, maybe Korea. Considering they harbour so much power in them and how coocoo bananas everyone went to gain the Dragon's Heritage power, who's to say the people at the origin of it aren't as well? Maybe the country went into complete disarray with its disappearance? I think there's a lot to explore left. The other endings don't work as well, of course.
Then there's also the possibility of a prequel with Tomoe as the protagonist. She's made out to be powerful samurai that's only ever mentioned in passing. She's the only one who could almost defeat Isshin. I feel like there's something you could do with that premise.
But I'd have to agree that Bloodborne 2 would make a lot of sense. Dark Souls 4 would kinda undermine the narrative portrayed throughout the series, ER is also pretty much finished, I feel like.