[FairyKing]

Yeah, I want more female riders as much as anyone (probably more than a lot of people, honestly), but none of the women in Saber really seem like they should be Riders yet.

Luna has been lost in who-knows-where, probably some part of the Wonder World, and we really do know nothing about her. If she were to show up in the present as a Rider, the show would have a lot of explaining to do about where she was and how she became one. Which could be really satisfying, the writers are foreshadowing Touma rescuing her, so it would be a great twist if by the time he met her again, she had grown up fighting for her life against Wonder World monsters and mastered a Seiken just to survive. Especially if she held a grudge against him for failing to keep his promise for so long. But there's no indication that the writers will do that, it's just me making things up.

Mei would have to get way less panicky and more competent. Which could also be really satisfying, she has shown that she wants to help, and if it was done well having her become actually useful could go a long way towards making her a more likable character. But it's not the kind of thing that could just happen in one episode, and if the writers tried to rush that it would probably just fall flat.

Sophia is the leader figure to the Sword of Logos so she clearly is supernatural in some way, and I wouldn't be surprised if she has some kind of power, which could even include a Rider form. But since she hasn't shown anything like that yet or gone into battle, that means that if she does have that kind of power, she must be saving it for when it's really needed. Even if she does reveal that she's a Rider, she'll probably have some reason for almost never transforming.

That only leaves Buff Tinkerbell. And let's be honest, calling her a character at all is a stretch, she's really just a finisher special effect that happens to be portrayed by a live actress. Plus, she can beat up monsters just fine without being a Rider. So as hilarious as it would be to see her become a proper character and get a Rider form of her own, I highly doubt that will happen.