Because I have this preference, I'm always looking at new rules that could make the Cleric more interesting or unique. One idea that found its way into my head was inspired by one of the miscellaneous sidebars littered throughout the 3rd Edition version of Unearthed Arcana: Spontaneous Domain Casting.
For those of you who aren't familiar with the mechanics associated with the d20 version of the Cleric, I'll elaborate. Each Cleric chooses two domains associated with its chosen deity. These domains grant them additional spells they can memorize and a few unique abilities. The Cleric may also spontaneously cast either cure or inflict spells by sacrificing one of their prepared spells. Which kind of spells they can spontaneously cast depends on the Cleric's chosen alignment.
This proposed house rule would change that. Instead of spontaneously casting cure/inflict spells, the Cleric may spontaneously cast the spells associated with their domains by sacrificing a prepared spell of the appropriate level. For example, a Cleric with the Fire and Sun domains could spontaneously cast burning hands or endure elements in place of a prepared 1st level spell.
While the cleric would receive one less spell slot per level (the extra slot they receive to prepare domain spells), I like how it makes a player's choices even more important. The domains you choose now determine what spells you can spontaneously cast and these spells help make each cleric more distinct. Clerics who just so happen to worship the same deity might not just have different domains, but can utilize different spells as well.
When I was running 3.5, I probably would have been very hesitant to utilize this house rule because the "spontaneously casting cure/inflict spells" rule allowed Clerics to prepare more varied spells while still being able to pull out a healing spell in a crisis. However, Pathfinder Clerics receive the Channel Energy ability that can be used to heal people, so I'm not sure it would be as detrimental to switch out these rules there. Because of that, I might just utilize this house rule for my future Pathfinder campaigns.
What do you think? Do you think it's a good idea to allow Clerics to spontaneously cast domain spells instead of cure/inflict spells? Do you think it might cause some unforeseen problems?