|Art by Luka Rejec|
- They're tied to items.
- No level/class restrictions for use.
- They're often unlimited in use.
- Usually their effects "just happen" rather than calling for a Save.
5e's Spells are pretty tame by comparison, but rather than a total rewrite you could focus on the final bullet point and make everything feel a bit more impactful.
- Any spell that allows a Save to completely negate its effect now automatically succeeds.
- If there's a complication (typically something that gives the target Advantage on their Save) the target gets to make their Save as normal.
- Saves that allow the target to "shake off" the effect afterwards are applied as normal.
- Accept that this a super dirty solution and is going to mess up the careful balance of spell levels.
- Cantrips are the real danger here, but I don't think it would be a massive issue at the table. The mild-option here is to keep Cantrips are they are but I can't bring myself to recommend that.
- If you're worried about the boost in spellcasting power, remember it works for enemy casters too.
Charm Person now automatically charms somebody as long as you're not already hostile towards them. The more interesting payoff here is that the target still knows they were Charmed by you, so you've got to brace yourself for the consequences of Charming a powerful individual.
Fear this works, because they still get their Save to shake it off when they get to safety, after which they're presumably coming for you.
Acid Splash d6 damage to two nearby targets isn't all that bad, smart opponents will break formation.
Unaffected Spell Examples
Sleep is based off hp, so no change here. There's still the chance that nothing happens here, but it's relatively low if you plan correctly.
Grease is an interesting one, because its primary effect is to coat the ground in grease, with the DEX Saves for anybody passing through happening as a result of that. I'd leave it untouched, as even if nobody falls over, your Grease spell has still had some impact on the world.
Finger of Death for cases like this where the Save is to avoid half the damage, just leave it as-is. It's a bit weird when you've got other spells causing auto damage, but this whole thing was always going to screw with balance.