Leiopelma

There is of course no precise link between popularity and playability, but I think it does play a role. I doubt that Kokonoe would have ever become playable for example if fans hadn't been asking for it for years. Fanart isn't an exact metric, but it is suggestive. In this popularity contest, made a few months after the start of the manga, Mai didn't even make the list. I doubt she did much better in the more recent ArcSys popularity contest, though I would be interested in hearing her exact ranking if someone knows it. Of course preexisting popularity isn't a requirement for playability; after all if that were the case no new characters would be created since they have no established fans. But if an existing character seems unpopular, the game developers might decide that adding a completely new character instead would do a better job of attracting buyers.

My guess is that it will end with volume 5. Ending the manga in 2 chapters from now would be too abrupt. There are too many things that need to be resolved; from Mai's relationship with Taro and Shiori to what happens to Albar, as well as Relius' and Hazama's plans. I could only see it ending in volume 4 if the magazine Remix Heart is published in decided to axe the manga. I'd say that Mai could potentially become playable. With her previous combat training and new power boost, it would be plausible for her to take on members of the main BB cast on a roughly equal footing. And from a story perspective, Kagura is looking for her to secure her help in overthrowing Izanami so she has an in-story reason to appear in the next game. Conversely though, she doesn't appear to be that popular; she only has 15 pieces of fanart on Pixiv tagged with her name, while BlazBlue as a whole currently has 27376 images.

Jubei already had his cameo in chapter 11. The manga is progressing pretty fast. I thought it likely that Mai's friends would eventually find out that she was a man because Noel in the new BlazBlue manga complained that "even Mai" had bigger breasts than her, regarding that as particularly unfair, but didn't expect it so soon. I guess that since the male characters are unaware of Mai's true identity it will be easier for a romance to develop with Taro since there no need to worry that he might be repelled by Mai's previous form. I wasn't expecting the protagonists to find out that Albar was evil so soon either. It seems that the manga overall might be drawing to a close. Though Relius and Hazama have still not acted in Torifune yet despite saying that they would do so, so that is something that needs to be resolved.

I don't think that Sumeragi has anything against fanservice; she used to make Blazblue yaoi doujinshi after all (her favorite pairing appears to be Ragna/Jin). I wonder if she was hired to work on Remix Heart after people at ArcSys saw that work. (I do find it rather amusing that her design for Hazama in Remix Heart is identical to the one appearing in the porn she made.)

Sumeragi posted on Twitter the cover of the next Remix Heart volume. She says that it will be released on May 9th.