Neutral Heroes

They don't have to move, and they can appear after the stack is attacked but before battle starts, but if so, include the ability to use Scouting or Eagle Eye or something to predict what the hero might/will be and certain skills/Gladiators etc. Heroes can be restricted to "Custom from Set" and custom heroes (with primary/secondary skills/spells/Gladiators) can also be added as Random from Set options for the entire map, or for certain monster stacks or guardians.

Grey-flagged heroes were my dream from the very start. They cannot be anything that a parties standing still though.

Yes, it would be hell to provide the AI and the support for neutral wandering creatures or heroes. Practically impossible at this stage. But neutral guardian heroes (like what H4 has) would be awesome, even better if you could get any custom neutral stack, whether with or without a hero, to guard any specific map location before it can be used (or if starting off flagged or flagged by an event or script, before another player can use it but the owner of the flag can use it before the guardians are defeated).

I've created neutral heroes by accident when playing with the map editor. I think they might have already had this behavior. If so, it would make sense to turn it from a bug into a feature.