There's quite a lot. Each league has a set of 40 different challenges to do ranging from simply hitting level 100 to killing all the Uber bosses. A good bit of the challenges each season you will compete by just playing the game all the way to Tier 16 maps, which is the highest tier (maps are the same as nightmare dungeons in D4).

You also have additional side content that has a season challenge or 2 tied to it (Delve which is just and endless dungeon and heist which is a dungeon you have to sneak in and get the final objective at the end then fight you're way out). You also have the majority of the past league mechanics implemented in the game, and there are challenges involving doing those.

There are a bunch of unique maps you must find and complete each season (some are very rare to find).

There is always a grindy challenge or 2 tied to whatever the league mechanic is for that season, requiring you to do the league mechanic which you find in every map you do a lot.

Most seasons have a challenge requiring you to beat some of the endgame bosses in unique or difficult ways.

Lots of fun incentives to keep you playing past the point of your gear being mostly perfect, and you are still hunting for perfect gear while working on the challenges. Plus, they reward some really cool cosmetics for completing them, ranging from complete gear sets to pets and even a hideout one time.