i think i'm gonna be surprised if the game doesn't take place in modern age. the reason is that i'm betting that MS doesn't know how many more Castlevania games they're gonna make in the future, and they want to tell a complete story. if they don't end up making any more past LOS2, that'll leave us hanging at LOS1's epilogue.
i think the three LoS games together will tell a complete story. Gabriel defeating Lords of Shadow and Satan [LoS], becoming Dracula [LoS DLC], getting beaten by four Belmonts at 100 year intervals [MoF], "giving up" (because he knows he can't win, he's not as evil as he used to be, he just wants to die, and/or he's just waiting for the Belmont Clan to die out) and stays in a hole for 500 years [LoS Epilogue], and then fighting Satan for the off chance that he'll be able to die [LoS2], and probably getting redeemed in the end [LoS2 or 3].
any games that come later would then be a sequel to LoS2 in the new era, or a prequel, or fill some gap between the three games.
of course, who knows how many games MS is planning on making, but that's what i'm going to expect.