Honestly... They can just use Traits to upgrade existing skills into new, better ones.

Heck, this can also aid with sync too, if they push the actual base skills into the lower levels, then trait them into better skills at the higher levels.

So for example, instead of Dork Knight being trash before level 70 when they get The Blackest Night. They instead a lesser version of it at like level 30, The Black-ish Night. Then when they hit level 70, the trait upgrades it to the full power version. Maybe at level 80 they get an even more potent version idk.

Though, not everything needs to be just potency increases, some skills can gain additional functionality (With the same overall usage), such as how Warrior's Berserk becomes Inner Release.