Allow specific spells to be reversible, as in PnP.
Set it up it similar to spell immunity.
As an example. You memorize cure light wounds. When you attempt to cast it, it pops up a selection bar that allows you to select Cure light wounds followed by a target cursor, or inflict light wounds, which begins auto-casting (since it's a touch attack). It otherwise remains the same as now, except you didn't have to choose between them at the time of memorization, which is actually accurate to PnP.
Also raise dead would be reversible to Slay Living. Heal into Harm.
It's basically the same as what 3rd edition did with clerics, except in 2nd edition, it wasn't exclusive to clerics. Some arcane spells were reversible, and druids, paladins, or rangers could as well. Unless a kit or mythos specifically forbade it, any caster with access to a particular spell could reverse it. (this would ALSO apply to some special abilities, such as gained by the Bhaalspawn).
The spell resources would remain largely the same, merely requiring a new core spell named after the beneficial effect to anchor them to, the core reversible spell would simply have descriptions for both effects. This would also free up spell slots that are currently tied up from having to choose between spells that in actual DnD can't be memorized separately.
Arcane Reversible spells (Or spells with multiple options that currently aren't)-
Fire Shield (Cold or Fire) All summon elemental spells (choice of Ice, Air, Fire, Earth per cast) Flesh to Stone (Flesh to stone or Stone to Flesh) Symbol Spell (Death, Fear, or stun) Imprisionment (Imprisonment or Freedom)
Divine Reversible spells-
All Cure X Wounds spells (Cure X wounds or Inflict X Wounds) Heal (Heal or Harm) Raise Dead (Raise dead or Slay Living) Remove Fear (Remove fear or Scare) Neutralize Poison (Neutralize Poison or Poison) All summon elemental spells (As above) Holy Smite (Holy Smite or Unholy Blight) Holy Word (Holy Word or Unholy Word) Greater Restoration (Greater Restoration or Energy Drain) Resurrection (Resurrection or Destruction) Symbol (as above)
There were several other spells that either weren't implemented at all or only 1 part was implemented.