I would think either Clint Eastwood (as he's already got the custume) or an MJF character (Scott Howard or Alex P Keaton).

I would say Clint Eastwood. Jennifer would find it very sexy.

I believe he'd actually look like Part I Marty dressed up in his Part III costume. I think he'd look even cuter there. Too bad we won't ever be able to see that - unless someone knows how to manipulate photos enough to provide a very convincing picture.

Wha do you think of the idea of Marty dressing up as another MJF character?

People would probably think that they're in a Michael J. Fox movie.

To me, though, it seems like a neat idea for Marty to dress up as another character that Michael J Fox played. Either Scott Howard or Alex P Keaton, since those are both characters that MJF played in the same year as the first BTTF movie.