Sorry if this has been suggested already, but I finally got around to watching all of the Firefly episodes this last week along with Serenity and a realization suddenly hit me. Mal Reynolds is basically the same character as Han Solo. Sure Mal is a bit more witty and sarcastic, but they are very similar characters otherwise.
With this in mind I tried to envision what Solo would of been like if Fillion portrayed him, and the idea seemed pretty cool. So I was thinking that it would be cool if thy cast Fillion as Han Solo's son in the new trilogy! I assume that the movies will be set in the era with that generation of characters, so he would be almost perfect in that role in my opinion.
Also, yes I recently read that both of Han's sons have personalities that don't really match with this idea, but chances are they will say screw the expanded universe canon and go with their own, so why not this idea?
TL;DR-Cast Nathan Fillion as Han Solo's son in the new trilogy.