I think there are true rising stars in the announcing world. The problem, IMO, is simply that Fox would rather have Joe Buck, who won't say anything bad about anyone, versus a guy who will say that umpires are wrong or that a player did something bad.
I think that John Miller would be pretty darn good if he didn't have Joe Morgan killing the dick out of his broadcasting.
As a Red Sox fan, I get to listen to Don Orsillo for ~150 games a year. I like him. He knows baseball and he generally has pretty good insights. Even at his worst, he's far better than anyone else on TBS or Fox. I'm sure there are good play-by-play and color guys all around MLB.