In the map editor there should be an option for alternative types of bridges with different terrain than open, at least for ice and desert. It seems silly to have 'open' terrain bridges on an all ice section of the map for example.
Interesting idea! Due to the amount of programing / graphics etc needed its not something we can add quickly Ill make a note though and see when/if it can be added.
Having 'Ice' terrain would be neat. Allow water and snow creatures to move over it and get their respective bonuses. Thematically speaking, the water units would swim under the ice,and the land units would walk over it. Then you could build bridges out of ice that both land and water units could occupy.