Permit me to quote myself here...
Increasing Hero count
The scenarios have set player counts that can limit the playability for some groups; you can increase the number of heroes that can be used in a scenario. Be wary about doing this as it can obviously unbalance a scenario, but it is not impossible to do (and better to do it than have people sitting out or you being forced to only play a few scenarios).
If you are going to increase the number of heroes, then follow these general guidelines:
Increase total Overlord gems by 2
Increase Overlord gem recovery by 2
Move 2 extra gems into each hero fatigue box at scenario start
Specific Scenario Changes
On top of this, certain Overlord characters in some scenarios should probably have their wounds increased (especially if they are the objective):
Hunting the Tigress: increase Skuthus and Zaporavo wounds by 1
In the Heart of Darkness: increase Skuthus’ wounds by 2
Infernal Pursuit part II: increase Zaporavo wounds by 2
In the Name of Ibis: increase Skuthus wounds by 2
The Crawling Shadow: increase Thog wounds by 2
EDIT: additionally, you may find that adding a few more minions to already existing groups can help offset the heroes additional 'killing power'. If a group of minions is under 5 then add an additional figure to that group (place the additional figure in the same space as an existing figure of that colour).
I hope you find something that helps/is interesting in here.