- 1 spell per magic-user level
- No more than 2 spells per spell level
- Spell competency = level / 2 round up
- Progression stops once there are 2 slots per spell level
Thus you get a progression that looks like this:
- Easy to remember
- Compatible with the standard magic-user class
- Approximates traditional gameplay at low levels
- Only starts to diverge at 4th level
- Scales back at high levels
- Never more than 12 prepared spells assuming B/X
- Exactly one new spell slot gained per level
Unless you already hate the traditional Vancian spell system, I don't see how this isn't an improvement.
This would also work for spells of up to 9th level (Supplement I: Greyhawk/AD&D style), if that's the way you roll (just add three columns, and extrapolate to 18th level).
Addendum: this actually works reasonably well for clerics, too. Just round down rather than up for spell competency, and stop at the highest leve of cleric spells. So first level clerics still wouldn't get a spell. Access to the highest level cleric spells would be pushed back slightly.