Personally Id like to see Revan return as the main character. I want to know what happened with him/her when he/she left to wander the unknown regions.
As for returning characters, Id like to see Mandalore return. So you can further help him rebuild the Mandalorian clans. Who knows maybe invade another planet with a team of Mandalorians.
Id like to see Visas return as well. She was pretty cool.