If the DLC really happened, and they actually released, say, a Castlevania stage, characters, songs, and trophies, these would fit nicely:
Songs:
(insert the boss theme from the first few boss battle of Portrait of Ruin here, since M'aiq can't find the blasted thing on youtube)
(insert Theme of Simon, which Rasto already posted in another thread)
Trophies(seperated by differing timelines/origin of appearance):
Simon Belmont Death Dracula --- Soma Cruz Julius Belmont Cilia Fortner, Dmitrii Blinov, and Dario Bossi Hammer Yoko Belnades --- Alucard Richter Belmont Maria --- Jonathan Morris & Charlotte Aulin Wind(Eric Lecarde) Stella & Loretta Vincent Doran
Potential Characters:
Simon Belmont. He's probably one of the only guys that would work well without having 12 clones. Some of his alternate colors could easily be made to resemble other Belmonts, like Richter, Trevor, and/or Julius.
Jonathan & Charlotte. They could be an interesting tag-team-esque character, maybe have Down+B allow the player to switch between the two so extra attacks can be accessed. Final Smash could really be any of their Team Attacks. Holy Rain, Meteor Strike, and Dark Gate would be the best candidates, though.
Stickers:
Not gonna list any now, but almost every Belmont/major char could be used, as well as dozens of monsters and items from the series.
yesyesYES Dracula would make a good playable character, too, or at least a boss in SSE. They should have added more representative bosses... Also, a Death Assist Trophy would be good. Gaze Up at the Darkness from PoR would make a pretty good song, too. I'd post it, but the guy who uploaded the entire PoR soundtrack deleted it all.