What do you think the next starters should be based on? Should they repeat another animal, like a water cat?

Well, we've had several starters that "repeated" concepts, including:

Bird starters: Torchic, Rowlet, and Quaxly.(Piplup count?)

Turtle starters: Squirtle and Turtwig.

Crocodile starters: Totodile and Fuecoco.

Cat starters: Litten and Sprigatito.

Monkey starters: Chinchar and Grookey.

As far as I remember, these are the only ones that share concepts, and I know that technically the idea of the starters' final form is currently more closely tied to the idea of where the new region is based, but I still think it's cool to try and discuss it.