IMO all the page space a caster’s spells take up ought to also be functionally counted as space in their class entry, and martials ought to have a similar chapter full of established abilities for those classes to reference too.
While I would not require that myself (since it would imply that every class that can cast spells should have spell lists of equal length, for example, which I don't think is necessary--and likely harmful!), I certainly agree with the underlying conception.
Functionally, all of a class's spells are part of its class mechanics. That's one of the easiest ways to show just how powerful spellcasters are compared to martials. Like, for example, the Cleric--shortest spell list of all "proper" full-casters in 5e, aka excluding Warlock--has "only" 117 spells. If we presume each spell is roughly 1/4 of a page (which is a roughly-decent average), this means that the Cleric class does not have 9 pages. It has 9+(116/4) = 9+29 = 38 pages of mechanics. And don't even get me started on the Wizard! It has, not 11 pages, but 11+(242/4) =
71.5 pages of mechanics.
All compared to the Fighter's measly 9 pages. By this comparison, the Wizard has nearly
nine times as much mechanical weight to it as the Fighter does. (And no, I'm
not going to include the EK spells. That's literally "your Fighter chose to become part-Wizard"--you aren't getting more Fighter mechanics, you're literally becoming a crappy Wizard.)
Now, again, I
do not think it's necessary, nor even positive, to try to force every class to have identical breadth. I think that's a pretty serious error (and, because I know folks will jump on this,
no I do not believe 4e did this, BUT I would prefer NOT to discuss that further here please). But this analysis really does show just how much MORE you are when you play a Wizard, or a Bard, or whatever, vs anything like Rogue or Fighter.
There are, IMO, several ways to address this, some better than others. I would personally like to see, for lack of a better term, "Deeds of Derring-Do". Deeds require practice; you can't just trot out any Deed whenever you like, you have to be
prepared for the opportunity when it strikes. Deeds require triggers (most of the time, anyway): you can't just declare that you blind an enemy or whatever, the enemy needs to be sighted in a way that could be taken away, etc. I'm sure there are more do's as well don'ts for how to make this work reasonably, but I'd like to think it's entirely possible. Deeds are organized into Disciplines, reflecting the general...kind of thing those deeds do. Perhaps they can be tied to being trained in specific skills? That seems like a reasonable choice--and a great reason for Fighters and Rogues to have more skills than most characters!
Then, each innately non-caster class (Barbarian, Fighter, Rogue, and possibly Monk) picks a few Disciplines at 3rd level, and then one or two of those to unlock the deepest secrets therein at, say, 12th level. Innately part-caster, part-non-caster classes, like Paladin and Ranger (and possibly Monk), qualify for a feat (or perhaps a fighting style?) that lets them pick up a single Discipline, no advanced options. Classes that innately support combat, but require build-up, such as Artificer and Warlock, can spend other resources to acquire a Discipline (perhaps a one-time Invocation for Warlocks, and a delayed subclass feature for Armorer Artificers?), and again, no advanced options.
This creates a space of play that
casters have to try to dabble in, if they even qualify at all, and where casters simply cannot reach the highest heights. There's now a
reason to be a Fighter instead of a Paladin or Blade Warlock or whatever else: there are secrets of blade and body and bone that only those wholly dedicated to it can reach them.