I've never wanted to be an umpire, but I have thought about what my dream call would be:
Decisive game (e.g., CWS finals game 3). I'm the 3rd base umpire. Home team down 1. Bottom of the 9th. 2 outs. Bases loaded. Left-handed hitter. Full count...
Check swing...
Appeal...
...wait to make sure the entire stadium and the TV cameras are on me...
...let the tension build...
and finally:
The umpire last night pissed it all away.