The cleric of Desna in my campaign has multiclassed into Spherewalker, and I'm tempted to allow her to go past the regular 5 levels in it (if she so chooses). She could get to 7 or 8 by the campaign end.
Even though leaving BAB, Save and Spell progression as it is seems powerful enough, would you think of any other added abilities that the class would have post level 5?