If it's a complete new character, i'm gonna say Wol from Mobius FF out of the blue; i've played Mobius for quite a while now and i have to say he fits surprisingly well and has tons of potential for movesets and alternate costumes. Besides, Y'Shtola aside all new characters have been from spin-offs so far.
If instead it's a returning villain, i'm gonna say either Exdeath or Jecht. I'm still operating on the logic of having characters from chapters who have currently a stage (Kain and the spin-off newcomers being the only exceptions), and considering the II and XII stage were added only recently and XIII and XIV would mean an entire new character, i'm thinking of the only 2 possibilities left.
I was also tempted to guess Tifa, but Sephiroth was added relatively recently, so i dunno.