Suggestion = In melee maps, you could have, like, secret areas with crates or something in which there's gold or lumber.
Another Suggestion = Heroes! Not overpowering, spell-sporting heroes, but a remake of a unit with a name. No extra abilities, damage, health, mana ... nothing. It expands the possibilities of gameplay. Of course, they couldn't be used in melee maps.
Last Suggestion = Mobs/Enemies! It think, maybe, your already doing this? Anyway, you should, but the only reason to kill them is for gold (bounty, since your not doing heroes [no exp/special items]). Also, make a giant squid that lives in the water and attacks your boats!