One solution would be, to do non-magical options on a item level. Nonmagical Items, that can be used by martial to mimic or do even other stuff than magic can do.
You just need to advanced from medieval fantasy to renaissance fantasy and have a Leonardo Da Vinci run rampant with Fantasytech. You want a fireball equivalent for casters? Here is a powder bomb to throw! Lightning bolt? Here is a Tesla Coil to wind! (Or pretend, you can have charges where the martial has to charge condensators on a long rest in order to use items). You wanna fly? Here is an ornithopter.
The answer to too strong magic is always science. Science and advancement always level the playing field.
If your response to the thread title is any variation of "nothing" or "there is no problem" or "I haven't experienced this problem" or "I like that this is a problem," then this thread isn't for you. Please keep that comment to yourself and move on.
The premise of the thread is: in D&D 5E there is a caster / non-caster gap and casters dominate non-casters.
If you want to argue against the premise of the thread, then this thread isn't for you. Please keep those comments to yourself and move on.
So the question is: what specifically can D&D 5E do to fix this problem?
The two obvious broad solutions are varying degrees of nerf the casters and buff the non-casters.
I'm partial to the system in Fabula Ultima. You can create effects on the fly, as long as it fits with the theme of your "school" of magic. It can be very costly in terms of mana, depending on what you're trying to accomplish, and the GM can require an appropriate component.
That sounds kinda similar to my system! If you’re doing Animism (Commune) to call out to and treat with a spirit, it’s pretty obvious that you might need a name or an item from thier sacred place or related to thier purpose, while Alchemy (Preparations) will generally require at least one each of: a base, a catalyst, fuel, a container, and an external source of power. If what those should be isn’t obvious from context, you and the GM work them out on the fly based on symbolic sympathy and other magical principles of the setting.
This gives you broadly defined framework with easily understood limitations (what skills can do for free or at a cost is explained in how to play alongside explaining the resolution mechanics), allowing for a lot of quick and dirty improvised action and a lot of considered and even “optimized” ritual magic when given the time. Quick and dirty ritual magic is risky, and hard, and can have both short and long term consequences, but since any skill check can be Pushes to a Partial Success if it fails, it is semi-often still worth it to try.
Oof yeah. I’d also say, if your idea was how mana generally worked, I would be all for mana as the default. I despise D&D spellpoints. If I’m spending a resource to cast spells, a level 1 spell should cost 1 of the resource.
First, I think casters need some nerfs. Not many but some. I'd remove or seriously limit resurrection and teleportation magic as well as magic that allows easily rest, thus breaking the attrition model. And I feel the casters have too many spell slots per rest or the rests are too short. Making gritty rests the default would help.
Now as for the perennial, "martials need to be mundane!" "no, they can be mythic heroes and wuxia badasses!" I have said my solution many times: emphasise the tiers of play. They start as mundane, but evolve into mythic heroes than can do clearly superhuman stuff. And then just cap the campaigns you want to keep more mundane at level ten or so.
The biggest problem ist that almost no DM puts enough encounter per day against the party and most DMs are quite lenient and inconsequential towards magic users but use "real world" metrics to measure success of martials. I had one DM that let my rogue roll DEX checks to climb a freaking ladder (of course I failed, slipped down and got falling damage) while the wizard did get to pull of the most crazy actions with his spells (often stuff that is not even in the scope of the spell, but "creativity" bonus by the DM).
Let your martials be heroes too, let them get creative, without punishing them or putting them into additional challenges. I had one other DM who let me do crazy stuff as a monk and it felt great. "Can I run up the wall, kick off it, jump onto the dragon that tries to fly away and stun punch it out of the air?" "Is your movement enough?" "Yeah" "ok, than roll your attack". Thats it. No extra acrobatics check or whatever, he just let me do it. If you don't want to let that fly without an acrobatics check, than at least give your spellcasters the same treatment.
"Can I use Ice Knife to freeze the river" "Uhm, roll me first an Intelligence check if you can find the right form the spell to achieve that" instead of "yeah, why not, that sounds creative".
Let both magic users and martials get creative and heroic, or no one. But not only one of these charactertypes.
And yeah, fight more battles / encounters. Martials big plus is consistency, but they can only prove that value, if they hav enough encounter to be consistent.
My analysis of the problem is simple. Linear fighter, quadratic wizard hasn't gone away - and 5e has no conception of what being a high level non-caster means. A level 19 fighter does exactly the same things as a level 11 fighter other than having extra uses of abilities. The game works give or take up to level 10
My basic solution is therefore simple. Ban spells of sixth level and higherother than as plot device magic. I've several suggestions.
Hard cap at level 10. Most games end by that time anyway.
No spells may be learned above level 5 other than as quest rewards. Slots (including Mystic Arcana) can be used to upcast lower level spells known
E10 based on the 3.5 E6 variant
Hard cap at level 10
Anyone who doesn't multiclass gets all the level 11 abilities except sixth level spell slots. This is because of the break points and level 11 abilities are often very cool
After this point extra feats/ASIs and spell slots can be bought for XP; each costs more than the last.
Oh, I don't believe its possible for a high level martial to exist without supernatural abilities in a form I'd want in my game. It would require you either radically redefine what "mundane" means, or that you go narrative, neither of which are acceptable options to me. At some point you have to flip that switch.
If other people want to advocate for that, that's fine.