Thanks to Wayfare and PPR for this idea!
This rule introduces an initiative penalty for casting spells in combat and is apparently straight out of AD&D (though with reverse numbers).
Here it goes:
1. At the start of each combat round, all characters wishing to cast arcane or divine spells or psionic powers, as well as spell-like and psi-like abilities, must declare by them before any action starts, in reverse order of their initiative roll
. If the character wants to take non-casting actions before casting spells they must declare this intention.
Failure to declare spells or before-spell actions for a turn disallows them for that turn.
2. Each spell the character casts that turn is done so at an initiative penalty equal to the spell's level (including any modifiers to said level such as metamagic). If the character performs non-casting actions before casting, such as move actions, the initiative penalty is doubled.
a. Quickening a spell allows the character to add its casting stat's modifier to that spell's initiative.
b. A spell with no somatic and
no material component will take half the initiative penalty, rounded up.
3. If the initiative of a spell drops below 0 it will only take effect on the next turn. Any negative initiative on a spell also means negative caster levels strictly for the purpose of using that spell. If the caster is left without enough caster levels, the spell will fail altogether: the character simply wasn't ready to use such complex magic this time around. The spell slot will count as used.
Note: Substitute "spell" with "power" and "cast(er)" with "manifest(er)" for psionic classes.