Ohai everyone. Sorry I haven't been on in a while, but I'll try to be more regular.
What reason or scenario would you have for Dawn coming back to the show?
After Ash finshes whatever he wants to do in Unova, he goes back to Pallet town. Dawn is traveling through kanto for the contests when she meets up with Ash for a pallet town contest or something. Then Ash travels to the new region he's going to next time, and dawn tries to sneak along beside him and "travel" with him without him noticing. Someday whenever Ash's friends are all off doing their own thing, she springs up on him and tells him she wants to travel with him again. Then they go through this whole new region and do the normal region routine with moar Pearlshipping hints.