Try putting back the ellipsis:
1. Who do you want to fight Benny?
2. Who do you wanna fight, Benny?
And answering the questions:
1. I want Duke to fight Benny.
2. I wanna fight Duke.
Not that wanna cannot be used when want is separated from to.
Who is gonna fight Benny?
Duke is gonna fight Benny. ("going to" remains intact)
Who is gonna fight, Benny?
Duke is gonna fight. ("going to" remains intact)
And on:
Who is going up river to see Mary?
Jake is going up river to see Mary.
*Jake is gonna up river to see Mary. ("going to" is separated)
But, the sentence below works because "going to" is intact. It is an auxiliary expression there and not the present progressive.
Jake is gonna go up river to see Mary.