When you complete the three acts, you can access the endgame mode with all your characters. This game mode allows you to level up a new character without doing the campaign if you wish, by doing randomly generated dungeons. But mostly, it unlocks the endgame content and its rewards.
The endgame feature allows the player to construct buildings and complete projects for the City of Stormfall. These projects require resources that you will be able to gather by completing random dungeons with or without modifiers. The harder the challenge, the faster Projects can complete. As usual, completing higher dungeon difficulties will also grant you with more interesting rewards.
Once you complete a project, you obtain its dedicated reward and unlock other projects, which provides you with more and more interesting rewards, including additional bank tabs, talent points, components, dyes, and other cool stuff.