UngeheuerLich
Legend
Why not just make it an arcane tradition?
Give domain spells for the mystic theurge and maybe some armor proficiencies. Require one of them be prepared all the time. If you have cleric levels and chosen the same domain cleric and wizard levels stack for access. Every domain in the mystic theirge entry has a few wizard spells listed and level 6,7,8 and 9 spells from both lists.
If you don't want to be that restrictive, you have level 6, 10 and 14 wizard abilities you can use to emulate clericy things like turn undead and other domain abilities.
Give domain spells for the mystic theurge and maybe some armor proficiencies. Require one of them be prepared all the time. If you have cleric levels and chosen the same domain cleric and wizard levels stack for access. Every domain in the mystic theirge entry has a few wizard spells listed and level 6,7,8 and 9 spells from both lists.
If you don't want to be that restrictive, you have level 6, 10 and 14 wizard abilities you can use to emulate clericy things like turn undead and other domain abilities.