this is more curious speculation but, i wonder what DnD 6e would be like if it fully abandoned spell slots for a spell points/mana structure like final fantasy typically does, spell levels could stay pretty much as they are i think (you can't learn a 3rd level spell before you're fifth level)
this system can also be used for martials, with them typically getting less MP but their abilities costing less to 'cast', with their abilities drawing off stats or other fundamental properties that make them typically inefficient for casters to use, alternately you could make a split between caster MP and martial Ki, with the difference being more akin to petrol vs diesel, they both power engines but engines are made in a way they only work with one or the other.
I think it could work. Level Up uses exertion points, which only martials get, and are used to power
maneuvers. And adepts--which are monks, but renamed so as to be more encompassing--get extra exertion and use it both for maneuvers as a replacement for ki points.
So there could be Ability Points, and whether you use them to power spells, maneuvers, or other abilities depends on the class. Dunno whether or not the number you get depends on your class or your level, but I'd wager the latter since it would encourage too many multiclass dips if some classes got a really high number of points. Of course, you could make give points different names depending on class, so you can't use your Fighter points to power Cleric spells, and vice-versa, but that might make for extra bookkeeping.
What I could see with this is having fewer abilities but giving each one more options. This would be easiest with spells. The
Fire spell would give you an option of cone of flame (
burning hands) for 1 point and big explosive flame (
fireball) for 3 (or whatever). Maybe a similar martial ability would be (for example) move 15 feet (outside of normal move) and attack for 1 point, and getting bonus damage or extra move would cost extra points.
Keep the numbers low to make math easy and it'd work quite well.