I agree that Rya should come back, pretty much for the reasons given by Sucrose. As for reserves:

-Ruto- Amusing to have around, potentially useful, and I think it'd be interesting to see her developed.
-Evangeline- I'd be interested to see more of her, she's got a lot of potential as a character.
-Erika- More screen time to fine out what she saw in Terra's dreams; I'm so curious O_O
-Ala- Really like the character, and would like to see her evolve.
-Sandra- Still hasn't really figured it out yet; if you have ideas for that, I'd support her return.
-Angel- Subplots! (I like this one a lot, incidentally, and it ties in well to Seed)
-Eda- Once again, if you can I'd love to see how you develop her.
-The Ancients [Trilo, Chelle, Draca]- If done well, these could be really interesting/entertaining. If you think you can handle it, I'd be interested.
-Dusk- Venomoths make the best ninjas. Search your feelings; you know it to be true.
-Kai- Because he's a cool guy.