A year ago I wrote about advancing Vancian magic with spell checks and slightly flexible spell slots. This time, I want to combine these ideas and expand on them for a simpler improvement for old-school games.

First, let’s imagine you just want to house-rule AD&D for substantial improvement. Here, I’d recommend…

• Spellcasters can cast cantrips at will, but these consist of what 5e calls Prestidigitation for mages and Thaumaturgy for clerics and druids.
• Arcane casters:
  • You get Detect Magic and one additional 1st-level spell each day.
  • You can choose to cast a different spell from one you memorized, but you must save vs spell or suffer a minor wild magic effect.
• Divine casters:
  • You get one additional 1st-level spell each day.
  • You can choose to cast a different spell from one you memorized by first spending 10 minutes praying.
• Magic Missile: Each missile requires an attack roll at +1.
• Sleep: Those affected are not comatose, just sleeping. Doing anything that would normally wake a sleeping creature–including making noise or touching–causes all sleeping creatures to awaken with alarm.
• Fireball: It does 1d4 damage per level of the caster and has a fixed area of 20-foot radius.