How about having a half-human/half-Betazoid?
Maybe as captain trying to deal with the constant noises in his head. Gives the human experience and the alien one too, plus some comic relief.
As for the rest, they should be human or races which look like human(Budget constraints). Trill, Vulcan, Bajorans, anything which doesnt take too much makeup and an actor can get ready faster.
Why not have the federation go through a civil war, Starfleet Personnel and Federation citizens disenchanted with the "Socialist" paradise overthrow the legitimate gov't and seize power. They position a demagogue as ruler and they begin to go against all the principles of the Federation.
StarTrek: Redemption = great name, the redemption of the Federation and of the crew and maybe the whole quadrant?