Since greek fire spits fire. shouldn't the snowmen in our towns be weak to fire? it should make sense as the description for greek fires says it spits out fire, and snow is weak to fire. so could you implement this?
could add a flag o the greek fire for it to do extra damager to both constructs but also snow/ice items and then have the snowmen the snow/ice option.
To be honest, even if that would happen IRL, we have meet the videogame logic before. If you were to do that you should make them melt also ice obstacles and burn thorns, with would render their items useless.
However, adding some more traits to enemies and some bonuses to units to deal with them seems a really good idea.
An example: the clay golem "family" (i propose this term to refer to small,med and big monsters of the same kind) has only that monster. Let it be the small one, for the medium add a metal golem, with the trait "Metalic", trait that makes monsters vulnerable to shock weapons (sages and more to come) and add something along that line for big. Small would not take extra damage from sages, but medium, beeing made of something conductive, would. Of course i talk only about monsters, not other units. If one unit has the fire trait and in the future you add another unit that may burn IRL, it does not have affect damage... unless you like the idea so much you go on and rebalance all existing content... which, lets be honest, wont happen.
This can be used to make monster battles more interesting while not overcomplicating PvP.
PD: when i talk about units i mean trainable units, hunters, braves... and so on