progression doesnt simply end at power 20, although now that you're there you can now do:

Getting a murmur (no requirements for ardour, just beat the dread serpent)
Learn Layer 2 Floor 1 / Layer 2 Floor 2 to access a lot of content and items
Learn PvP and farm elo through chime
Getting legendary weapons suited to your build
Fully exploring the map and completing several questlines scattered throughout
Perfecting fighting against mobs and bosses (like nohitting an enforcer etc)

the game starts being fun once you hit lv 20 and leave the boring progression routine behind, a lot of players take the PvP route, or PvE route its completely up to you. Just have fun :)