I don't think the next game has to necessarily follow Gabriel.
Actually, Mercury Steam could pull a Metal Gear Solid 2 here.
First chapter, you get to play Gabriel in his new state; couple hours of him going back to Castlevania and reclaiming the castle for himself, taking the name Dracula and becoming the serie's villain; foreshadowing and NPC introduction, and probably him subduing Zobek/Death for good.
Cue Chapter 2 - something like 99 years later, a young guy called Belmont... and you know how that ends.
They really got the chance to play the 100 years mechanic good this time around.
Next game could be about explaining how Gabe becomes Dracula, how the Belmont clan comes to be and how the two are tied; would probably still be medieval in tone and style. Probably still medieval in tone. But I'd totally be up for the series actually showing the passing of time, a la Assassin's Creed. A XV century episode would look radically different, and I wouldn't mind an *actual* Victorian episode.
Heck, I'd take the Castlevania structure in a direction where half the game is made of chasing Drac or his minions across Europe, until you get to the actual Castle for the final showdown. That thing is alive. Make it move around. Make it change with the times. And for the 1999, DO pull a DMC3. Castlevania popping right into the middle of central park. The forces of evil taking over - full scale world vampire war, and a single Belmont infiltrating the castle to save the day. The castle would look gothic in 1999 too, and the whip would still be the only way to take out Drac.