Nice game prototype I could easily see a story mode where heroes go bragging the crowd in front of the dungeon where you'll beat em easily :D
Some ideas to improve the overall experience
- In my opinion it's a bit slow, it should be more nervous such as "Dead Cells" for the example. Very controllable and reactive.
- Maybe it could be cool to choose which boss you place at which level to let emerge some strategies and like giving malus to heroes when dying so they could be easier to beat for the biggest ones.
- Use a progression in vagues. So you'll get a wave of x heroes with growing stats/power as you did. Then you'll beat the wave and it start with another hero type.
- When beating a hero or completing a wave you'll got level-up or skill points to enhance the dungeon or the boss skills.
Well I must say as simple the idea is it's quite inspiring to make it grow :)