TwoSix
Everyone's literal second-favorite poster
I would say either a meaningful feature (say, a central class feature, upgrade to that feature, or a capstone) or a meaningful choice every level (whether that be from a class specific menu or a general menu, like feats) would be ideal.
I'd note that gaining multiple new spells at a new spell level is equivalent to a "meaningful choice", to me, but just gaining a new spell or some new spells in a spellbook isn't quite enough.
I'd note that gaining multiple new spells at a new spell level is equivalent to a "meaningful choice", to me, but just gaining a new spell or some new spells in a spellbook isn't quite enough.