Not that I play AD&D anymore, and haven't since the late 80s, but here's an old rule that came up in discussions at my last GURPS game. Comes from playing GURPS with a guy who played AD&D with me in junior high school.
Spellcasters can only memorize one of any given spell each day. If you have three first level spells, for example, you can't take Sleep x 3 or Magic Missile x 2 and Charm Person. You've got to take three different spells. So Sleep, Magic Missile, and Charm Person is fair, but you can't double and triple up.
IIRC we made an exception for clerics and druids, so they could take extra healing spells if they wanted, but otherwise, they were included too - no Heat Metal x 4 for the Ranger/Druid or Flame Strike x 3 for my cousin's cleric, but Cure Light Wounds x 2 was just fine.
The goal was to force some versatility and expand the spells used in game. Otherwise everyone just maxed out their Fireballs or Lightning Bolts, Melf's Acid Arrows and Magic Missiles, and Heat Metal and Warp Wood spells. Borrrr-ing.
I won't claim to have originated this idea - If you notice the AD&D Conan modules did this to all the spell casters, and I'm sure I read it at least one other place, too.
It worked okay in play, too, because people did choose some interesting spells.
So, submitted for your amusement and interest. Maybe you'll like it.