I think this game's mechanic would benefit a lot from having the option to unlock and use "modifiers" on the runs.
Another game that does this really well is "GemCraft: Frostborn Wrath" (check for inspiration).
But in general, the idea would be to gradually unlock different kinds of modifiers that you can "activate" on the runs. Like:
- Monsters with more HP (2x, 4x, 8x)
- Monsters spawns more often (2x, 4x, 8x)
- Bosses spawns more often (2x, 4x, 8x)
- Monsters more aggressive
- etc...
This would make the game more challenging for endgame players.
(obviously, if you add more modifiers to the run, you get more rewards... gold, exp, items)
Keep working on the great game, and I hope you like my idea/suggestion.