There is a new mode which you can play. It's available near the play button in character selection menu. The mode, as the name suggests, doesn't have end. After you defeat first vawe of enemies you will be able to upgrade your character's health, movement and attack speed as well as cooldown speed, spells damage, dodge chance and damage taken reduction. After you are done witch upfrading you will get spell / item / super item so that you can withstand next round, which will be harder. You will have to face both normal enemies as well as bosses at the same time, but you can take adventage of traps placed around the map to finish them faster.
You can't win this, but you can try. It shall be fun. For me.