Calibur 4.0 has a very interesting stance. I don't want to give people hope by saying it's feminine, but it's kinda feminine in my opinion.

Man, I really like that Solomon Storious's face is the back of an open book, that's so cool.

It also looks like we are getting some collective legend rider wonder ride books as well 1st book is about all of the Showa riders. 2nd is about heisei rider and the third is about the reiwa riders

it is and you can also tell by the torso, it's very narrow, the arms and legs arent bulky either, i actually knew from the zero-one real time pic for zero-two to zero-one, that zero-two was feminine and it was izu just from the body structure of the suit.

It could mean nothing, but I'm also looking at how the Riders are spaced out and the only two Riders in close proximity to one another are Sabela and Durandal and Saikou and Calibur, which leads me to believe the identity of Calibur is someone who is particularly close to Yuri.

My money is on Tassel, since the feminine-like pose definitely fits his mannerisms, though the next episode could easily debunk this. Mei is probably the runner up after that, though it'd be really odd seeing the most upbeat character on the show become a Rider that's been traditionally associated with characters stuck in perpetual suffering lol

Mei would be a good choice to become Calibur because the wielder of the Kurayami is forced to see possible outcomes and Mei has the power of positivity to resist the doom and gloom. She is incorruptible.

Kurayami: "Let me show you the dark futures that await you MWUAHAHAHAHAH"

Mei: "LALALALALALALALA LALALALALALA NOT LISTENING"

Kurayami: "IMPOSSIBLE NOOOOOOO"

*Kurayami is now perfectly controllable*

Either Mei or Luna could be the next Calibur!

Luna is a child, and at this point, there is no way they’re hiring a new actress to play her as an adult (especially because, judging by the last episode, ‘adult Luna’ would just look like Sophia, and so be played by Rina Chinen).
Realistically, the options are Sophia, Mei, or Tassel, depending on whether or not he survives the next episode or not.

Tassel's probably gonna die. Either Mei or Sophia would be nice.