NHF – new heroes for Heroes V 3.1.

Yes, all of this can be done. The main thing is to avoid problems like the one with Falmer. Special abilities that trigger events for all heroes are not very good: they can negatively affect others at any moment. I initially didn't want to make Eymund like that.
Can you give a more specific example?

For example, the reward could be calculated based on clearing a treasure vault "one level higher." Of course, in the case of treasure vaults at the maximum level, we would have to introduce some kind of special tier for the hero. Another option is to increase the reward by a percentage (but this would create a problem with artifacts).