Spontaneous Domains (3.5e Variant Rule)

Spontaneous Domains
Cleric domains feel unsatisfying. They are exceptionally powerful in the sense that they give you a bonus spell every level, and they expand your spell list (for up to one spell per level per day) to spells you wouldn't otherwise get. But that has problems. Some domains are completely awful in this framework because they give you spells you already have, thus not expanding your list at all. Furthermore, you could have prepared that spell in a slot you already had and used a different spell that you wouldn't otherwise have in your free slot.

I feel as though domains are a mechanic that Clerics see as an afterthought, rather than an integral part of their class. Thus, we can use the following class feature to replace the Cleric’s domain class feature:

Deity, Domains, and Domain Spells: A cleric’s deity influences his alignment, what magic he can perform, his values, and how others see him. A cleric chooses two domains from among those belonging to his deity. A cleric can select an alignment domain (Chaos, Evil, Good, or Law) only if his alignment matches that domain.

If a cleric is not devoted to a particular deity, he still selects two domains to represent his spiritual inclinations and abilities. The restriction on alignment domains still applies.

Each domain gives the cleric access to a domain spell at each spell level he can cast, from 1st on up, as well as a granted power. The cleric gets the granted powers of both the domains selected. The cleric does not gain an extra spell slot to prepare these spells. Instead, he may choose to spontaneously "lose" one of his currently prepared spells in order to cast one of his domain spells of the same spell level or lower. This works similarly to the Spontaneous Casting class feature.

Analysis
The addition of spontaneous casting can be seen as a power-up, and I would not argue the point. However, the Cleric also loses one spell slot of each spell level, which should not be taken lightly.

Under this variant, Clerics are not as discouraged from taking domains that contain only Cleric spells. They will not get the added utility of additional spells on their list, but they will gain the ability to spontaneously cast any useful spells from their domains. If there is a spell that is particularly useful, they may consider taking the domain even if it does not grant out-of-list spells.

Use With Other Rules
This variant interacts okay with Mana-Based Spellcasting. Clerics already gain a great deal of spontaneous casting under that rule. Use of these two variants together means that Clerics are not giving up anything to gain the extra spell that they could cast spontaneously. You could still be okay with taking domains that contain mostly spells from the Cleric list, because it's just more free spontaneity.