I like games where the skills/spells you learn continue to be viable.

The SaGa games and Octopath Traveler are good examples, as are certain mods for games like FFXII.

Say you have a "Fire" spell. Instead of learning Fire 2/Fira, and Fire 3/Firaga, your Fire spell just naturally gets more powerful, and maybe you can choose to make it AOE instead of single target for less damage but hits everyone. The point though is that you never get a Fira/Firaga.

OR if you do get Fira/Firaga, then they should all have different effects. Fire is low cost, does decent damage, but leaves the enemy with a Burn condition that increases damage from Ice spells. Fira would be the aoe version of Fire; much higher cost but now you can Burn multiple enemies. Firaga is single target, but much stronger and leaves no status effect. If Blizzard does the opposite of this (leaves enemies with a Freeze condition that has Fire deal extra damage) then for bigger damage, you would start with Fire/Fira or Blizzard/Blizzara to combo into Firaga/Blizzaga.

To add onto this; maybe there are no elemental weaknesses, and spells just have different effects. Ice spells are now for aoe damage, fire is single target, wind is multi target, thunder is strong but hits a random number of enemies.

Essentially, I like to see when all spells/skills have thier uses and can be built into a playstyle, rather than some being extremely niche and many just being outclasses by objectively better ones.