I'd like a mana series crossover as well.

Something from Legend of Mana I'd like is a Dubear minion

Or the elementals as minions from the mana series, Shade, Wisp, Salamander, Undine, Jinn, Dryad, Gnome, Luna.

And also Chronotrigger. (Another game with visual designs by Akira Toriyama who did Dragon Quest) Chrono Cross not so much. I didnt really get into that game cause the story felt kind of meh and most of the characters felt like fillers to me.

As for the thing about female character costumes. With the exception of Snow all the males you listed were flagship characters of their respective game. Lightning was the flagship of her game. Yuna was sort of a co-flagship of FFX as that story revolved around her and Tidus, though she was essentially the star of FFX-2 afterwards.

Cecil would be the flagship of FF4
FF1 was the WOL which is essentially a nameless hero with a generalized appearance
FF3 was originally centered around 4 orphans but I think that DS 2d/3d remake they did painted Luneth as the main character. I don't think they actually had individual appearance visuals until that DS remake. However this title I believe was the first to introduce the job system which made character appearances change with the selected job.
FF5 was centered around Bartz but made use of a job system so the characters could repeatedly change appearance depending on their selected job.
FF6 Started off with Terra though that one I feel is hard to paint a specific character as the main since the story shifts focus at multiple points in the game. It is easy though to paint various characters into a minor supporting role story wise. It was even possible to lose Mog, Umaro, and Shadow permanently during the game if you didn't do certain things during the first half of the game.