So stagewise, if there are more stages than characters, we need to see at least 5 more stages, for a total of 26.
We have not seen Garfield, Jenny, Rocko, or The Beavers stages. That takes care of 4 of them.
Theory time: the last missing stage belongs to Gerald and stage theory was meant to be a thing, meaning Lincoln would've been next in line to return.
It makes sense with it having multiple characters for Hey Arnold to get another stage. Likely the return of Traffic Jam due to it's popularity for casual play.