TL:DR: To retain the feeling of spellcasting having your limited powerful spells and a weaker baseline to fall back on, both spell slot progression should be slowed down, and total spell slots should be reduced.
We're talking about spellcasting, but I promise, I'm not going to talk about Martials, and I'm not even going to bring up much about power levels or whatever. I want to talk about what I think is a fundamental design flaw with spellcasting, its relationship to the Adventuring Day, Multiclassing, and how the feature changes in terms of gameplay experience as the game goes on.
The level 1 Experience and Everything after
So you've just made a level 1 Cleric. You start your adventure and go up against your first encounter. Everyone's at the top of their game, and the first fight doesn't seem tough. You stick with your Cantrips and beat the encounter without too much difficulty. Encounter 2, however, is tough. You end up spending both of your Spell Slots. For your 3rd and final encounter, you're back to relying on Cantrips. Your party makes it through the day and gets ready for the next one.
A couple of these and you hit level 2. An extra spell slot, how welcome! Your adventuring days go on, and you've got the same loop. Cantrip when you can, bring out the spell slots when you need to. But as you level up more and more, things change. Level 3 gives you a massive additional 3 spell slots, with 2 of them being a new spell slot level! Level 4 gives you another spell slot, and level 5 not only gives you 2 new spell slots of a higher level, it also upgrades your Cantrips! All throughout this, you learn additional spells.
What's happened to our gameplay loop? At level 1 and 2, we conserve our limited spell slots and use them when the situation calls on them. As the levels grow, however, there's almost a division in how we're expected to play. With each new spell slot level, we gain greater ability to bring out the big guns. This makes sense, as the challenges we face grow. But we also gain more lower level spell slots. This would suggest that we're pivoting from using Cantrips to conserve spell slots over to using lower level spell slots to conserve our higher level spell slots. At the same time, however, our Cantrips are growing more powerful. So our Cantrips are still expected to be used to conserve spell slots?
Breaking the Adventuring Day's Back
So what's the big problem here that I call a fundamental design flaw? Well, even without going further, we seem to be undecided on how a Spellcaster should play. We get more and more spells, and our cantrips get stronger, and we start being able to sling spells around all day, a stark change from level 1 and level 2. That's not necessarily a terrible thing, but there are three elements that take this design decision and use it to completely decimate the Adventuring Day.
- Concentration: Concentration is a mechanic used as a balancing tool to limit powerful spells (as well as elegantly limit how many lingering effects can be present at once). You have to maintain Concentration when you get hit in order to maintain powerful effects. The issue is that now, your spell slots are getting a lot more mileage than they should be. A difficult fight might only require one impactful Concentration spell! Your limited big guns remain don't necessarily feel limited when all is said and done.
- Low Level Power: Okay fine, your big spells are strong and impactful, but you do eventually run out of them in an Adventuring Day. Whether it's Cantrips or lower level Spells, surely the gameplay experience of using your free or less impactful spell slots to conserve your big guns stays relevant, right? Well no, because your little guns aren't necessarily lacking in impact. You still use the same Save stat, so your lower level spells aren't less likely to work. And oftentimes, your lower level spells can have enough power and utility (especially utility) that you can very safely conserve your higher level spell slots.
- Too many slots: Remember, your quantity of spell slots continues to grow throughout the entire game, but the Adventuring Day does not. The threats get bigger, but you're not going from 3 deadly encounters to 5 deadly encounters to 10 deadly encounters. The turn count of a day isn't massively increasing to keep up with your increasing quantity of spell slots. If there is a sweet spot in terms of number of slots, we surely pass it.
Put all of this together and we can see the problem. We can stretch our spell slots increasingly wide throughout an Adventuring Day while retaining consistent high impact. Our early level experience has fundamentally shifted. We're no longer conserving our high impact abilities. We're always high impact! The Adventuring Day would have to massively shift in order to keep up with us, but that would also affect everything else, like HP and Hit Dice, and all the non-spellcasting classes.
To fix this, I'd suggest a full on rework to spell slot progression. And I actually think it works better for how a spellcaster progresses in universe.
The table
| Level | 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th | 9th |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | ||||||||
| 2 | 3 | ||||||||
| 3 | 2 | 1 | |||||||
| 4 | 2 | 2 | |||||||
| 5 | 2 | 1 | 1 | ||||||
| 6 | 2 | 2 | 1 | ||||||
| 7 | 2 | 1 | 1 | 1 | |||||
| 8 | 2 | 2 | 1 | 1 | |||||
| 9 | 2 | 1 | 1 | 1 | 1 | ||||
| 10 | 2 | 2 | 1 | 1 | 1 | ||||
| 11 | 2 | 1 | 1 | 1 | 1 | 1 | |||
| 12 | 2 | 2 | 1 | 1 | 1 | 1 | |||
| 13 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | ||
| 14 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | ||
| 15 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| 16 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | |
| 17 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 18 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 19 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 |
| 20 | 2 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 1 |
The goal here is just to keep up the expected experience. Your ability to throw out super powerful spells is there, but you can't just toss them out. A hard fight at level 11 forcing you to use a 5th and 6th level slot leaves you in a pickle for the rest of the Adventuring Day. Your high level spells allow you to be an incredibly valuable member of the team, but play too aggressively and you start to feel like you contribute less and less as the game goes on.
This also curbs two common issues: your out of combat utility is a legitimate sacrifice to your combat effectiveness. Throwing out a Knock spell can absolutely come in clutch when needed, but that's one less slot for an encounter. Secondly, it keeps even your lower level slots from repeatedly getting value. You get two Blesses in a day, without spending one of your higher level slots on Bless.
As for why this works logically, your magic isn't suddenly jumping exponentially. At even levels, you're working to increase your ability to continue using magic. At odd levels, you work to further your hone your magic. In world, you increase your magical stamina, so to speak, and once your stamina can support more magic, you work to increase your potency.
Multiclassing
So I've been recently designing with Multiclassing in mind. Not dips, actual Multiclassing. In contrast with Extra Attack, which dissuades multiclassing by removing a high impact feature from your progression, Spellcasting dissuades Multiclassing by growing non-linearly. You don't just gain a new spell slot level when you level up. You gain more spells in total! You also learn more spells, increasing your versatility, which is a large contributor to overall power. Losing a level in a spellcasting class lowers your power in several different ways, which is why you rarely see non-dips. We've now kept is so that your power is mostly growing in impact, not consistency.
Matching the power of spellcasting is a lot easier now, and means that, if you wanted to make a tempting Fighter/Wizard multiclass, the Fighter doesn't need to have absolutely busted features to match another Wizard level (this is just an example, you'd still need a pretty great Fighter Feature that can compliment or aid spellcasting for it to be anything close to a thing).
Conclusion
No way around it, this is a massive nerf on a spellcaster. By the end of the game, we've gone from 22 total slots at 20th level all the way down to 13 total slots. Yes, we can still last through the Adventuring Day, but you can really feel your power go down heavily after you use your big guns. Are you still in the running for best classes in the game? Yes. Those clutch moments when you drop a perfect Wall of Force, or have Spirit Guardians screw over the enemy side, or have Spike Growth effectively cut off part of an encounter. But those clutch moments don't happen all the time now. Just like those first few levels, you need to be smart.