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The "Anarchy" tournament series quite clearly demonstrated which elements are gameplay-defining, i.e., overpowered, and which are not.
Overpowered elements that significantly distort gameplay:
1) Necropolis hero > all
2) Diplomacy > all, except for point 1
3) Free Scroll of Death Dealer/Fly - a reason to play a tight map
4) Cartographer - a reason to win a moderately tight map
That's basically it; the impact of all other banned elements on the outcome of the game does not exceed the typical unevenness of the map.
Various Solmyras and Dimers are mainly needed in the first week to quickly defeat tough guards or to fight unpleasant archers. I don't remember anyone building a game strategy around them, and I've read a lot of reports...