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Personally, I don't like the "magic mirror" spell. It would be better if, at the expert level, it could be cast on all units, even if only with a 20% chance. And instead of killing the entire enemy unit in "sacrifice" at once, it should only kill a part of it, which could then be resurrected with its health (taking into account the hero's magic power). Armageddon should be a 5th-level spell.
Regarding skills: it would be cool to have a hero with an ability related to "literacy," allowing him to teach 5th-level spells. And, for example, "eagle eye" combined with "scouting" could provide an enhanced effect (their vision remains, but it is only applied in battle), and so on.