...ask your GM since there are no rules for going beyond 20. Well, if your campaign is mythic you could add 10 mythic levels but that is more of a side upgrade rather than a linear advance from level 20.
Really, continuing to campaign at that level means being able to enjoy the capstone ability of your class. The only growth you should experience is the acquisition of goods, allies, followers and wishes.
But if your going to dip classes...
There are a few I think could be very interesting, but it's hard to say without knowing the build, I'm assuming high intelligence, but did you focus more strength? Or Dex? Did you dump charisma?
Both vanilla wizard, and vanilla fighter should suit you regardless, but if you didn't dump Dex, a paladin could synergize amazingly well. You may even be able to pull off a rogue or go into some prestige classes like dragon disciple
Also What kind of weapon do you use?
Honestly just post the build, it will be easier to give good advice :D