Geomancer requires arcane and divine casting, but you only advance as an arcane caster.
Mystic Theurge, True Necromancer, and Arcane Hierophant are the only three I know of. I believe the Arcane Hierophant has druidic prerequisites like shapechange. I think there is something with the True Necromancer [Evil?] that makes it unsuitable to most. At least I haven't considered it in so long, I have forgoten why.
I assume you are looking for something to take between level 15 and 20, there are some 5 and 7 level prestige classes with unique abilities. They are too varried in styles to make a suggestion. It may be worth finishing your spell progression into epic levels for some unique ability.