So after Scooby-Doo 2: Monsters Unleashed, there should be a Scooby-Doo 3. In it, Scrappy should return as the main antagonist, making a deal with Jonathan Jacobo/Evil Masked Figure to get revenge on Mystery Inc. However in the end, Jonathan Jacobo will betray Scrappy and take over as the actual main antagonist, gathering his own monster minions and Scrappy's former minions to kill Mystery Inc AND Scrappy. Therefore, reforming Scrappy and re-earning the trust and love of the gang.
As well, Scrappy will be revealed to actually be a puppy, despite Velma's claims in the first film.