If I could blow off a half an hour:

Bitterspice did hers. I'll do mine.

If I could be a scientist: I'd head to South Korea to do stem cell work.
If I could be a farmer: I'd sell out and move to the city. I've no head for agriculture.
If I could be a musician: I'd go on tour with the Rollins Band, just for the stories.
If I could be a doctor: I could finally afford health insurance.
If I could be a painter: my cousin, the painter, could develop a sneering hatred of me.
If I could be a gardener: see the farmer sentence.
If I could be a missionary: I'd spread the good news that God is dead.
If I could be a chef: I could finally work with my own Tandoor.
If I could be an architect: I'd rebuild the White House in an early medieval theme to better fit its current inhabitants.
If I could be a linguist: I'd definitively isolate the Seattle accent.
If I could be a psychologist: I'd subject Michael Bay to my Michael-Bay-Film-Aversion protocol.
If I could be a librarian: I'd tell the FBI to fuck off.
If I could be an athlete: I'd be the starting SAM linebacker for the Oakland Raiders.
If I could be a lawyer: I'd try to secure legitimate legal hearings for the GitMo inmates.
If I could be an innkeeper: I'd...yes, dear, I'm doing it, dear!
If I could be a professor: I'd be the mighty famous bigshot who's always on sabbatical.
If I could be a writer: I'd be me.
If I could be a llama-rider: I'd name him Floyd Flewellan the Llovlley Llama of Tywysogaeth Cymru.