Right, I don't think clerics should have to prepare their spells. I know thats how its always been done, but its never made sense to me that divine casters have to pre-choose their miracles.
I think there can be holy rituals for certain powerful long term effects, but generally speaking I think a cleric should just have a small pool of divine spells based on their deity that they can choose to cast on the fly. Like the 3.x Sorcerer (or the 3.x Evangelist).
As the base default, you could do worse than make all arcane casting traditional D&D casting and all divine casting on the 3.* sorcerer. That gives your "specialty priests" right there--every divine character is encouraged to be a specialists by definition!
I'd like more options, rapidly, since I'm not sold on the idea that such different mechanics won't have issues. You only have to look at the 3E sorcerer verus the 3E wizard to see them. But to get started, they won't be that bad, and something like the 3E cleric needed to be toned down a bit. Meanwhile, if the wizard is getting somewhat handled by something such as their idea that spells in low-level slots don't scale all that much, then it might work.
Personally I think that the 1 Night = Recovery needs to die. I have less objection to Vancian casting than I do to the wizard recovering everything every day. I don't think I'd mind Vancian Casting if the wizard required a weekend at a library or lab to prepare his spells. That makes it not an arbitrary restriction.
It also enhances rather than impedes tension. You need to make it back to safety to be able to reset your spells and you don't know how long that will take. (Unless teleporting but I digress). It prevents hot-swapping spells as under the classic vancian model, 8 hour rests are somtthing everyone needs. It better matches the fiction where spells or at least powerful ones are rare and valuable (I'm absolutely in favour of cantrips for all mages) rather than something you reach the end of the day and burn off.
I could make that work, but I'd rather have something a bit more flexible, as the default, than no restocking out in the field, no matter what. If nothing else, something like the Mongoose RuneQuest retaining of spells (in their divine casting system, ironically enough, the one truly "fire and forget" option in MRQ): Make an appropriate skill roll when attempting to "restock" for each spell. Make it by enough, you get the spell back. Fail badly enough, and it is gone until you get back and spend considerable time to restore it. Somewhere in the middle, you can try again tomorrow.
That is simple enough. Make a roll for every spent "Vancian" spell to see if you can get it back. Then people can vary the DCs up and down depending upon how close they want to get to traditional or more of what you advocated--with "always make the roll" or "always fail the roll"--so no need to roll--as the opposite extremes.
Then more complex modules can add options for switching out spells, feats that let certain kind of spells come back more readily, modifiers to the roll for good or bad conditions, etc.