What about the idea of having the possibility to adjust the line-up. E.g. in Boss levels you often have 2 or 3 enemies or 1 surrounded by 4 which could have different advantages.
- 3 heroes get + 66% health each
- 1 surrounded by 4 has the advantage that the 1 couldn’t be hit directly
- other variations could change the number of fields where heroes could get a hit
I am sure there are many other ideas