Or, some games you don't need to even find the runes. Dungeon Master is like this; if you know the runes for a spell, you can just cast the spell. This also allows the player to guess spells; it's not too unlikely for a player to accidentally cast Fireball when trying to use a basic light spell because the player accidentally clicked an extra time.

There's also Rudra no Hihou; to learn a spell, you just enter a mantra of up to 6 katakana characters, and every mantra corresponds to a spell. Then, with clues the game provides and some experimentation, you can find mantras that yield better and better spells. Also, some spell names from other JRPGs can be used, and some of them are suspiciously similar to the original effects. (For example, using the name of the sacrifice heal spell from the Dragon Quest games will give you a sacrifice heal, and Reraise (from Final Fantasy 6) is also suspiciously similar to its FF6 counterpart.)

The way robots actually work is as follows:
* Robots can equip normal equipment. Doing so gives robots stat bonuses, and they're different (and typically much bigger) than the bonuses that humans and espers get (plus, robots get stats from everything, unlike humans and espers).
* Robots are not subject to body slot limitations. For example, a human or esper can only equip one suit of armor and one helmet at a time; robots do not have that limitation.
* In SaGa 2, robots don't learn special abilities, but said abilities aren't needed (humans don't get them either, and they're less powerful than just using weapons or spellbooks). They do, however, recover item uses at the inn (though only to half the uses that other races get), and such weapons include things like machine guns that can hit entire groups of enemies.
* SaGa Frontier has robots learn special abilities from enemies. There are also circuit boards that they can equip; some allow special abilities to be used if the right ability is equipped. (The number of slots the robot has for them is based in its intelligence; it should be of no surprise that circuit boards typically increase this stat when equipped.)

I wish more games would take approaches like these to character growth; the level/XP system really has gotten old and heavily overused over the years, and there's so much design space that hasn't been explored here.

Yeah, I loved that system. On the other hand those runes make for powerful rewards for the player, like in Ultima Underworld. They can be a big incentive to explore properly or tackle an especially difficult enemy - or maybe break and enter a wizard's home ;-)

I guess the issue is that XP/Level systems are tried and trusted, and fairly easy to balance. And also, if many games suddenly did it differently, many players would complain that they want a more classic game again...

The system being easy to balance is, perhaps, a point, but I still feel that there should be at least the occasional game that breaks away from it, and that doesn't seem to happen. (The only recent indie game I've found that does this is a free (gratis) game on itch.io called Rxcovery that I only found out about because of a twitter thread.)

Of course, there's also skill point systems that suffer from the same problem of being overused, but the difference is that, unlike XP/level systems, skill points are not easy to balance; it's too likely for there to be situations where the game gets frustrating or impossible because the player misplaced their skill points.

Theoretically speaking, crafting them with different parameters (power, mana cost, duration,range...) . I know some games have that but I nevver played one extensively. In my mind, customization = good and RNG = bad. Though a system like this would also sacrifice some balance, especially if you have Bethesda-style playtesting.

Buy them from a store and let the actual methodology of learning be left up to the imagination of a player.

If spells are not a purchasable per the game/system, have them come as learned upon level up.

Variety! Different ones work better (mechanically and in lore/story) for different games.

So long as none of them are too grindy (especially certain outliers -- looking at you FF5 red mage). The less RNG frustration involved the better.

A good case for this is Castlevania HD (Harmony of Despair; the stage-based replay game...) that I desperately wish were available for PC. It runs the circuit of inherent and learning different ways for different characters, all in one game. But really it's a compilation game of characters from disparate games.

I know this is a non-answer, but it really is. I've enjoyed games with inherent learning, I've enjoyed games with pay to train, with various sorts of skill trees, with "equip to learn", with gear-only, and even with plot-based development. And I've played in each that didn't work so well. So long as the individual design is done smartly and integrated well, it can be successful. FF Tactics had a couple abilities learned by being targeted by it. And it suuuuuuucked and was frustrating. One of them was a summon.

FF5 was basically "broken" if you got White Wind and Mighty Guard early.

What if spells can't be purchased, and the game doesn't use an xp/level system?

Except that you can't get Mighty Guard early.

Also, I believe Mighty Guard is useless in a low level game because you don't have enough MP to cast it, and White Wind isn't generally worth using over Cure/Cura under LLG rules either (you don't have enough HP for it to be useful).

I could mention SaGa Frontier, where different races have different rules for learning skills as well as different rules for stat growth. Also, different types of skills have different rules (weapon techniques have one set of rules, spells another, and monster abilities a third).

Then I dearly hope they're somewhere in the overworld, because as we've sustained, the diceroll RNG school of learning isn't my jazz.

Research the spell like in Age of Wonders or Warlock.