Mana Points, d20 Style
Rule #1: Everyone casts spontaneously. It helps to just drop the sorcerer class outright, and call the main arcane caster anything you want to: wizard, sorcerer, mage, magic-user, it doesn't matter. Not using psionics with this system is highly advisable.
Rule #2: Spells cost MP to cast, 1 MP for cantrips/orisons, and 3 x the spell level for everything else. Metamagic, instead of raising a spell by +1 level, raises its MP cost by +4 MP (or +8, +12, +16, whatever) to compensate for the fact that you're not slowing down the casting time while casting spontaneously.
Rule #3: Characters who can cast spells from level one begins with 3 + ability mod known spells, and earns 2 free spells per level after that, even divine spellcasters. Everyone else (paladins, rangers, prestige casters with their own list and progression instead of just "+1 per level", etc.) earns 2 spells once they start casting and 2 at every level thereafter.
Rule #4: Major casters (wizard, cleric, druid) begin the game with MP equal to their relevant stat (INT or WIS) and gain MP equal to their stat's modifier at every level. Minor casters (bards, paladins, rangers, prestige casters) gain that modifier in MP as soon as they start casting, but only on levels where their caster level rises. Since bards and assassins, for example, raise their caster level at every level (including level 1), they would gain their CHA or INT mod in mana points at every level. Paladins and rangers, on the other hand, start casting at level 4 and only raise their caster level every other level, so they would get their WIS mod in mana at levels 4, 6, 8, and so on.
Rule #5: Chrono Trigger spell combination. Okay, you lost me. I can't honestly think of a good way to do this, short of some wimpy "aid another" action that adds +2 to a spell's DC for every extra person casting the same spell. Brand new spells and flashy effects that can only be cast by multiple characters are something you'd have to come up with on your own, perhaps a special school of high-level spells that are lower level and cost less MP when cast by extra people.