Anyway, currently there are no known restrictions on being both a PoH and GoL. Kairi could be the guardian of a PoH, while at the same time being cared for by yet another guardian, like Sora or Riku. Certainly she isn't going to just sit back and watch from the sidelines again this time now that it's official that she will be swinging a Keyblade around.
All that said, Kairi's actually in a rather difficult situation right now. While we don't want her to receive the damsel-in-distress treatment again, we also don't want her to suddenly be truly overpowered and on par with Sora and Riku (it takes experience to be at the same level as them, and Kairi's certainly lacking in that department). This all stems from the fact that she had been severely shafted in the past games and the concern that there wouldn't be enough time for Kairi to develop properly in KH3.
The writers will be having a "blast" dealing with Kairi in KH3. Here's to hoping that they will be able to do it well.