Well I accidently posted this in the R.Mika thread, but anyway what character would make you want to purchase SFV? I heard a lot of people saying "add X character and I'll be there day 1." I am still buying SFV, but Guy, Urien, or Yun would make me want to buy it more...
Just about anyone Capcom would let progress into a new stage of development. Gym Teacher Sakura, Professional Karin, Gentleman in a Suit Blanka... whatever. Any character can be made interesting with a bit of development.
I'd enjoy Elena and Makoto and Rose returning, and... Dhalsim and Dan, but I'm actually quite interested to see how the new characters turn out. I hope they continue to fill in some gaps in character design and archetype. We've never had an older female in a SF game before, for example...