I think at this point we can consider Bloodstained a success -- especially as I've seen more in the gaming press about Bloodstained than all of the post-Dawn of Sorrow Castlevania games put together (low bar to reach, but still).
It got me thinking about how Iga might "finish" his run on Castlevania without making a Castlevania game in name in a move similar to Bloodstained, and finally give us that 1999 game while doing something original enough to spin into its own thing if desired.
The title might be something that would communicate the "spiritual finale" vibe, like "Vampire's Eclipse" (good name for a franchise!).
Keep Dracula as the main antagonist (he's public domain, so Konami can't gripe). Have Julius Belmont become a Captain Ersatz of himself with a name like "Jules Schneider" (keeping a surname associated with the Belmonts seems like a decent idea, after all), who fights with a combination of whip combat and gunplay (being a 1999 setting and all) with magical augmentations for both.
To keep it somewhat distanced from Castlevania for lawyer reasons, the whip would have a different emphasis, being more of a tool and a navigational device than a weapon, though it also would be good to give it offensive uses as well. The best part would be making the Castle's map identical to the one in Aria of Sorrow (with completely new navigational paths, art design, and enemy placement, obviously). That would just be one of those little touches that would connect this game to Aria as a sorta-kinda-ish alternate franchise prequel.
The story would be an adaptation of the heretofore unelaborated-upon 1999 Demon Castle War, with all the exclusively Castlevania details removed or altered just enough to get it by, with a release for PS4, Xbox One, and PS Vita (guy can dream dammit, and Bloodstained has given me hope).
If sequels developed, I'd love to see them move further away from Castlevania and grow into their own thing-- a new branch on the Castlevania family tree, as it were.
Not really sure where else I'm going with this, but I guess I thought I'd put this stuff on the table and see where the discussion heads.