Why would it be bad ?
1- Turn will drag end for LOOOOOONG before it is up to you to play.
2- Too many heros mean that there wont be enough opposition, nor favor for everyone. Keeping one hero avaliable to dedicated minion hunting or totally Blocading influence spawning to the Western Area of the map (with the haven there are many stop gap).
3- One more heros means one more area cleared of influence by turn, and one less area to put thoses in.
4- Just adding an action for Sauron wont be enough, you will need to give him upper limits.
5- Peculiar chain action will be avaliable for the heros with more map coverage and increased ability to quickly gather favor and concentrate them when needed.
6- More rest action (potentially) means Sauron markers will perhaps advance faster.
7- Game will have to be rebalanced and cards rework.
8- Hero victory by clearing Minions, Monsters or Gathering Favor will be tremendously easier. Sauron win by Shire or 3 Complot Cards tremendously harder
9- Increased difficulty for Sauron to play plots because of less influence on map, less minions or monsters at his turn start and less characters not consulted by heros turn end.