Besides boats and ships, trains (I mean "real" trains with realistic 1:1 scale, not minecarts) should be added to the game. Because they can be useful when carrying resources (using wagons/railcar) from different climate zones and can reduce considerably the amount of time you spend walking from, for example, the jungle to the arctic.
Also, it can open a lot of new possibilities on servers. Because you can start your own transport system, and move resources or even people and sell goods.
As a reference, I recommend taking a look at the Minecraft mod called: Immersive Trains.
PD: Sorry if is a duplicated topic, but at least I didn't find a post about trains.