I've been thinking of the next Pathfinder character I have to make, and I decided on a Mystic Theurge.
After doing some research, I decided to use the Ecclesitheurge cleric archetype to satisfy the divine requirement, and Arcanist for the arcane requirement. I have not yet looked into domains, subdomains, arcanist archetypes, or feats because I want to know if this is a viable idea before continuing.
From the Ecclesitheurge:
At 1st level, when an ecclesitheurge chooses his cleric domains, he designates one as his primary domain and the other as his secondary domain. An ecclesitheurge can use his non-domain spell slots to prepare spells from his primary domain's spell list.
From the Mystic Theurge:
A mystic theurge can prepare and cast spells from one of his spellcasting classes using the available slots from any of his other spellcasting classes. Spells prepared or cast in this way take up a slot one level higher than they originally occupied.
And finally, the Arcanist:
An arcanist must prepare her spells ahead of time, but unlike a wizard, her spells are not expended when they're cast. Instead, she can cast any spell that she has prepared consuming a spell slot of the appropriate level, assuming she hasn't yet used up her spell slots per day for that level
Will this combination let me use arcane spell slots for domain spells?
For example, suppose I have Fire as my primary domain. Am I able to sacrifice a 4th level arcane spell slot to treat it as 3rd level divine spell slot (via Mystic Theurge), to then prepare Fireball via the Fire domain's domain spell?
Conversely, am I able to sacrifice a 2nd level cleric slot to prepare and additional 1st level wizard spell (such as Magic Missile), which is then usable via my arcanist's spells/day?