My group is just short of advancing to Silver level and we were thinking about mixing and/or substituting several cards from the overlord decks from Well of Darkness, Tomb of Ice and Altar of Despair. I know the SoB rules allow for treachery upgrades but we would really like to add some of the environmental effects (ice, mud, corrupted terrain), spawned monsters (a Golem would be nice) and traps.
My question is: would the addition or substitution of of 10 - 15 overlord cards from the expansions significantly unbalance the game? That certainly is not my intention and my card choice would not be driven by a desire to get an advantage. It just seems a shame to have all the expansions but not be able to use them fully.