As much as I like Soma, a third Castlevania rep that isn't Alucard just wouldn't make any sense to me.
Yeah, to be honest I don't see another option. Unless they decide to add another Belmont as an echo fighter, but with some unique moves to justify the inclusion.
I always shill for Alucard but Julius Belmont is actually my favorite Castlevania character, but he's not gonna make it unless what I said above happens. He has some unique cool stuff but I doubt Sakurai would even plan to give him a fully unique moveset.
Alucard is great, but Soma and Shanoa are great as well. Though Alucard definitely makes more sense, he's probably the most popular character within the series, and with people not that familiar with Castlevania he's definitely the most well known one, I'd dare to say. Mostly thanks to Symphony of the Night's popularity and the Netflix show.
It also helps he is the recurrent character in Castlevania with most appearances, excluding Dracula and Death, of course, which is kinda ironic considering he isn't a Belmont. He also was considered as our Castlevania rep at one point, according to Sakurai, which obviously means they would probably go back to him if they even have the chance to add another fighter, and his assist trophy model would mean a bit less of work for them.
Another cool option I feel people barely considers is Jonathan and Charlotte, though I guess they would kinda share their gimmick with Pyra and Mythra. In Portrait of Ruin you can control one of them, with the other one following you and attacking enemies, similar to the Ice Climbers, but you can also just with one of them if you want, and switch between them anytime. So that's a difference in comparison to Pyra and Mytrha.
Jonathan and Charlotte also could have some of their dual attacks from PoR, which are very cool. One of their dual attacks is them just summoning the "spirits", I guess, of 5 Belmonts, including Richter and Simon.