My theory for Sigmascape

[Nestama]

Name one naked girl in bikini armor we've fought pve.

The Ruby Princess! and if that's not a good enough answer: Sophia, Calofisteri, Garuda... excluding Siren and Succubus mobs because their whole shtick is to seduce.

Anyway, considering Deltascape had three bosses that weren't really all that memorable from FFV (then again, I've only played FFV once, so maybe they are memorable for some players?), I think it might be safe to say Sigmascape may do the same. Three bosses from X game and then the final boss being the final boss of aforementioned game.

I've already gone with my guess that it'll probably be FFX (since the gear is apparently of the Diamond series. Every character in FFX had their own exclusive diamond piece). I went with two random picks (because I had trouble with them as a stupid dumb kid) and then the final two being plot important.

Sinspawn Gui, Spherimorph, Yunalesca and Seymour (phase transition to Final Aeon in Savage).