Let players build bridges, please! I know that the problem of a river can be avoided with a boat, but it would be cool to see a bridge somewhere!
I think it could be both wood or stone based, for wood based it could be 8-10 boards and 4 wood blocks, for stone a stone based one it could be 30-50 stones and 4 wood blocks. Could require stoneworking for the stone one or carpentry for the wooden one