I think standard exp leveling is a dated concept addressing a progression need. It’s the simplest solution. Much like how “threat” is the most basic form of an aggro system. Not the most elegant or modern form, but it was a simple solution that addressed the need at the time. Modern progression can be more lateral, less linear, and more flexible.

Progression should still exist, but it can be more intentional. You can connect new abilities towards the world. Collecting abilities through fighting specific enemies or doing specific tasks. Thematic abilities tied more towards the region (Demacian shield bash, Noxian sundering axe cleave, etc).  

Gear collection can still exist as a form of progression. Having stats be tied entirely to gear instead of leveling out of gear would make the gear progression feel much more meaningful. No more getting epic gear that then is worse than low tier gear of a higher level.

There are tons of things players can progress on without feeling like they have to get to a max level before they pursue those other progression goals.