Honestly, though all of the magic variations out there are interesting, I'd like to see a reworked option for default magic.
How about a pure spellcaster class that has access to any & every sort of spell out there (arcane & divine, though it'd be treated all as arcane for the class) rather than being restricted to a single spell list? They could have it like the wizard class, where spells have to be researched/learned in order to use them, & they have to be prepared beforehand. Have this option as a replacement for all of the various spellcaster classes out there, or have it as the main spellcaster class, with various other spellcaster classes being more of themed specialists (like dread necromancers, beguilers, warmages, healers, warlocks, and even sorcerers in comparison to wizards).
Or, how about a paired-down core spell list option that provides generic versions of magics (notably attack spells), which can be simply modified by adding a theme/style to it. For example, having magic missile as a default 1st-level attack spell, which by default does force damage. However, why not allow a built-in option (rather than burning feats) where you can use a specific sort of energy instead (fire/acid/cold/electric/sonic), which provides additional benefits & drawbacks (maybe does a d6+1 damage instead of a d4+1, or has side effects like igniting flamable materials or continuous damage, but has the limits of not of being foiled by energy resistances, not working in certain environments, etc.).
Or how about a system that trades off degrees of success/potency for increased variables/limits? No-save spells or automatic-hit spells do the least amount of damage/have the shortest range/have the shortest duration, but duration/range/damage could be increased if the chances for complete success were reduced: making an automatic-hit spell more potent by allowing a save for half damage, or making it a ranged touch attack or even a ranged attack; increasing a spell's duration by allowing a save, reducing range, or giving it a HD cap; or even increasing a spell's range by allowing for a save or reducing duration.
Then again, how about providing rules/options for a more low-magic approach (if DMs want magic to have more of a mystique about it)? Maybe giving options for rangers, paladins, bards, & other partial-caster classes as non-spellcasting variants? Providing optional versions of the full-caster classes (druid, cleric, wizard, sorcerer, etc.) with limited 0-5th level spell access instead of the usual 0-9th level access?
What about non-fiendish Warlocks? Not just PrCs that provide the option, but full-fledged core classes? A divinely-powered Warlock (more akin to a cleric or druid)? A fey-powered/themed Warlock? A self-taught Warlock (not relying on any source for their powers, but just tapping the magic source directly & getting Invocations/Abilities directly)? Heck, how about a psionic version of the Warlock class?