If I went with a real person, then my sister pathvain_aelien - she has had to keep dropping her college math class (I forget what kind of math) because she has been having trouble with it and I couldn't help her with her tests, so if she takes it again, I could become her and take the tests for her. I'd have to hope nobody happened to ask me about anything LOTR or Harry Potter-related, though, because when it comes to those she knows way more than I do.
If becoming a fictional character meant being in their universe and P_A didn't take the math class again, I'd either be Mario and play through a Mario game in first-person (I wonder what 1-Up Mushrooms and stuff taste like?) or be a Homestar Runner character (I suppose I'd make a convincing enough Homestar or Homsar) to find out why there haven't been any H*R toons in a while and what it's like to pick stuff up with invisible arms :)
If becoming a fictional character wouldn't take me to their universe and P_A didn't take the math class again, then so far I'm out of ideas.