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the ability to choose an upgrade, I think it's a very good idea; now there will be more options in tactics!... it's interesting, will the choice be something like: either make a strongly attacking unit or a well-defended one?
goblins, orcs, cyclops - good,... it's unusual to see a wyvern with the Barbarians, but it's more appropriate than roc birds, because they are good creatures, unlike orcs and the like... so wyverns are more suitable than rocs...
centaurs would look good with the Barbarians, and in general, who said that centaurs are good and noble? and that they are more suitable for nature...?