I'd go with Clint Eastwood for the Trilogy Universe. His family has already seen him in the costume, and Clint Eastwood is a well-known famous person - so why not?
For non-trilogy universes, I'd go with Alex P Keaton or Scott Howard - as those are both characters Michael J Fox played in the same year. Oh, definitely Scott Howard in his human incarnation. I don't want that lovely face to be covered with all that hair.
Oh, why Doc? I know you're not the only one to have suggested that - but, as I mentioned at another BTTF Board, Marty might as well make use of that Clint Eastwood costume (especially as his family already saw him wearing it).
I just thought it would be funny if he dressed like Doc.
I guess it would be. Marty is so short, and Doc is so tall. I really do like the idea of Marty dressing up as other MJF characters, as well.