The Catholic priesthood is often used as an example due to it being the predominant religion in (the western two-thirds of) europe during the middle ages. It had its knightly monastic orders - which is what the current cleric class is based off, if I'm not mistaken, but these were separate - or at least distinct - from the priesthood in several ways.
My view of a Priest (not a monastic knight) is more akin to something like this:
01 Divine Channelling, Religious Specialization
02 Divine Aura, First Domain
03 Divine Guidance, Lay on Hands
04 Touch of Courage
05 Divine Grace
06 Touch of Vitality
07 Touch of Freedom, Second Domain
08 Blest Health
09 Touch of Sight
10 Touch of Health
11 Blest Body, Divine Favor
12 Touch of Blessing
13 Touch of Purification
14 Eternal Devotion
15 Touch of Clarity
16 Touch of Liberation
17 Third Domain, Blest Perseverance
18 Touch of Restoration
19 Touch of Life
20 Divine Emisary, Eternal Body
Code:
Spells per Day*[u]
LV 0 1 2 3 4 5 6 7 8 9[/u]
01 1 - - - - - - - - -
etc
10 4 3 3 2 1 0 - - - -
etc
20 5 5 5 4 4 4 3 3 2 1
* Not including bonus spells from a high Wis.
Note that all spell casters in my games have delayed progression, due to being a lower magic world. They do get other advantages, however.
Blest Health/Body/etc grant a bonus (based on class level) to various afflictions (disease, poison, etc). The various Touch abilities use increasingly high number of Lay on Hands points to heal various afflictions. Curing blindness (or deafness, for that matter) requires 8 points. For half again as many points (12) the priest can attempt to inflict such instead, but every time a divine ability is used there is a chance the deity will notice. And if the priest inflicts (or cures, for that matter) against their patron deity's wishes, it will go the worse for them (from failure of the ability to occur - despite lost Lay on Hands points, to perm loss of powers - until an atonement after a quest). With Divine Guidance the priest really has no reason to say they were unaware of their deity's wishes. (Basically a sense of dis/approval in regards to a single specific action, requires one full round of contempation).
So my idea of a priest (not playtested, as this is spur of the moment creation) basically is a delayed caster with a lot of healing type abilities and various resistances to physical ailments and those that might seek to drive them from their deity (eternal devotion). They don't get spontaneous casting, but then they they don't really need it.
I'm currently working on a new idea for casting spells, for clerics and other casters alike. Perhaps the idea of it will have some interest to you - or spark your own imagination for such.
When they prepare their spells they choose to refresh their memory on specific spells from the list of known spells. Then they can spontaneously cast from that (smaller) list. Each day they may change one spell per level (one 1st level spell, one 2nd level spell, etc) on their (shorter) list through this type of study / meditation / prayer. I'm playing with the idea of a means of changing a single spell later on with 15 min of prayer/study/whatever and perhaps a check (Will save? Concentration check? Spellcraft check?) - a sort of 'on the fly' means of alteration.
I'm not sure how well this system will work, but it is sort of an intermediary between the preparation system and the spontaneous casting system. The idea of one spell per level per day alteration means some forethought will be needed if they know a dungeon / encounter will occur in a few days, but the 15 min system might allow for getting that needed spell when really needed. I'm also considering the idea of - instead of 15 minutes - allowing the casting of non-prepared spells as normal but with longer casting time: one full round plus one full round per level, minimum. So a zeroth level non-prepared spell would take one full round to cast, a first level spell two full rounds to cast, etc. If a spell has a longer casting duriation than one round, the duration is increased (by the multiple of the level? perhaps doubled the longer duration?).
This entire alternate system has a several bugs to shake out (likely more than I am aware of), but I think it may have merit.