So once you complete the campaign you will begin maps. There are 3 tiers of maps, you have white, then yellow, then red. I consider mapping to be the endgame.
If this is your first build, the endgame will be rather punishing and you might not make it very far. It took me until my third character roughly before I was able to successfully push into red maps and beyond. There are special bosses to hunt down, and all the side content like Heist and Delve are very rewarding and fun.
Additionally you get another passive tree that allows you to customize your mapping experience however you want. Zizaran has a really nice video going over the endgame. It’s quite long, but very helpful. youtu.be/uf4wnvkS-mI