Finishing the adventure map makes it completely boring after doing it..!
To make it more fun / endless, there should be 4 game modes?
You start on easy first, then once you beat it, you move on to the next game mode.
Easy would just be normal monsters
Medium would have monsters with decent runes and relics
Hard would have monsters with medium features and ranked up a bit
Impossible will have hard features and have bonus traits
Each time you complete a game mode, the rewards get better? For example:
Easy mode island 1 : 1000 gold + 2500 xp
Medium mode island 1: 10,000 gold + 25,000 xp
Hard mode island 1: 100,000 gold + 250,000 xp + bronze relic chest
Impossible mode island 1: 1,000,000 gold + 2,500,000 xp + 5 bronze relic chests