Heroes 5.5 WarGame Edition - Tactical Edition.

Maybe it would be worth creating a permanent bonus for each hero to level the playing field with Humans, Mages, and the Undead.

I'm considering replacing the temporary bonuses received upon hiring with temporary bonuses received when visiting a town, i.e., so that, for example, a +5 attack bonus would be refreshed upon repeated visits after hiring. This would eliminate the need to hire heroes on the same day that the opponent plans to attack the town.

I don't understand, can you now build a Grail without finding it?

No. But the Grail, like many other buildings, will have certain building requirements, depending on the faction.