Necropooooooooooooooooost
Arcane Hierophant is a great PrC option if you take a Wiz 1 dip, it fully advances Wildshape (if you go for the Wildshape Ranger ACF), Animal Companion, and both Wizard and Ranger spells. It does rely on your DM ruling that your Wizard spells in the Ranger slots are Arcane, though. Works best with a backported Pathfinder ranger for better CL and Animal Companion progression.
Otherwise, if you're okay with less good spellcasting, Master of Many Forms is pretty great. Losing some fifth level spells/day is worth the kind of versatility you get out of 19-20 HD worth of MOMF wildshaping.