It would also make the support style classes obsolete, instead of players picking a support class they would instead just pick a strong damage dealing and just dual class into the support roles. Id immagine some of the combinations the heroes could come up with would totally decimate the overlords chances of winning.
That said, it could be an idea for a new campaign to implement something similar to your ideas. Atleast in this way it could be done in a much more controlled fashion. For example as a quert reward the heroes or overlord could pick up an ability.
Your idea of the plot cards being quest rewards would also mean FFG would need to go back in time and rewrite certain quests, not sure how they would do that They could certainly toy with some sort of similar idea in future campaign expansions however.