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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.
"Никто не уходит из героев вечно", - убедился на личном опыте, гоняю в сингле до сих пор :D