I think the one thing that keeps UTAU from combining phonemes like in vocaloid is the fact that UTAU was made in Visual Basic 6, a programming language that doesn't support synthesis like that via normal means.
I think it'd be cool to be able to skin UTAU. I mean, come on. A piano roll that's completely black and white, with blue, grey, white, pink, or yellow notes (depending) is kinda boring after a while.
Also, YES HAVING MULTIPLE TRACKS ON UTAU WOULD BE HELPFUL.
And I would like to see more flags. I love flags.