So far we know there is going to be a new girl and she is related to old lady munson and she is going to be Coop's new friend.
In addition to Fiona, there will be a number of new characters dropping by for the second season. Most will appear only in one or two episodes - like Phoebe's dad, a new favorite of mine - but some will stick around for more. Like the other alien cats. Hmm... maybe I've said too much.