[FairyKing]

Don't know why you're saying "officially," until we get any actual confirmation from Toei all we can do is speculate and wish. We can't actually choose anything.

That said, I think it's most likely to be Reika or a not-yet-introduced character.

But what I most hope for is for Mei to get some character development and take up Ikazuchi, since that would make her a better character (I do think she's been improving), it would mean we get to see Espada in action again without cheapening Kento's death with a revival, and it would fit with the ending.

I'm also a fan of the "Luna survived in Wonder World by becoming a Kamen Rider" scenario. I think I might have been the first to suggest it here. I'd especially like it if she resents Touma for failing to save her and isn't a true antagonist but is hostile to him at first.

It could be cool (no pun intended) if Yuki-san, Mei's boss who was transformed into the Yeti Megid, somehow became the Swordswoman of Ice, since that would give her the classic Kamen Rider backstory of an evil organization trying to turn her into a monster (which ironically none of the current swordsmen have).