Right now I'm kicking around the idea of using the Recharge mechanic to balance out the class features and spells. It's still in the development stages, with lots of tweaking and adjustments, but it goes a little bit like this:
- A "Recharge die" is an unmodified d8.
- You roll it at the end of your turn.
For spells and spellcasting:
- Spellcasters have one spell slot per spell level. So a 5th level cleric would have one 1st level spell slot, one 2nd level spell slot, and one 3rd level spell slot, for example.
- The Recharge number for a spell slot is equal to its spell level.
- When you cast a 3rd level spell, you can't cast any more 3rd level spells until you roll a 3 or higher on your Recharge die. If you want to cast Fireball again, for example, you will have to spend your 4th level spell slot, or wait for your 3rd level slot to Recharge.
- Each Reset roll applies to only one spell level. If you have multiple spell levels that are depleted, you choose which one Recharges.
- The Arcane Recovery class feature lets you Recharge all of your spell levels once per long rest as an Action.
- All spell slots automatically Recharge after a Long Rest.
- Cantrips are considered "level zero" for the purpose of this rule, and they automatically recharge at the end of your turn--no need to roll.
- 9th level spells effectively become "once per day" spells under this rule (you can't roll a 9 on an unmodified d8). This is a feature and not a bug.
For other abilities and features:
- The Recharge number for most features is equal to their equivalent spell level. A Tiefling's Hellish Rebuke ability, for example, has a Recharge 1 since it's a 1st level spell.
- The Recharge number for non-spell abilities and features is (8 - Ability modifier, minimum 2). A dragonborn with a Con score of 17 (+3) would have a Recharge 5, for example...after using their breath weapon, they cannot use it again until they roll a 5 or lower on a d8.
- If that dragonborn is also a sorcerer, and he has been waiting for his 5th level spell slot to recharge for three rounds before he finally rolled a 2, he has to decide if he wants his breath weapon or his spell slot to recharge that round.
Anyway, it's a work in progress. But the more I tinker with it, the more I like it.