For a classicvania, I think something close to
Dracula's Curse or
Super Castlevania IV's length wise is perfect.
Branching paths and secret areas could be a good way to increase the lenght without making it too long
Maybe adding some RPG elements like those in
Vampire Killer could work too, but always keeping them on the lighter side to avoid overdoing it or turning it into a Metroidvania.
cutscenes between stages like in Ninja Gaiden
This is something I would love to see. I know that
Konami didn't take
Castlevania's lore seriously on the first game, but for
Simon's Quest and
Dracula's Curse cutscenes like those would have been something really amazing.