This is basically a list of things I came up with while in game, which i think could offer some interesting game play changes. Others will be added in as I think of them.
Stationary Weapons: Basically, the ability to create (purchase?) certain hardware to help you fight, both of which have a very specific purpose. The first would be a artillery cannon, which would mainly be used for punching a hole through enemy defensive structure's. Cons include it's large size, and a ineffectiveness against mobile infantry. The second would be a olden style turret (Look up civil war gatling guns to get a general idea), albeit smaller. Very effective against infantry and small structures, but takes a while to start firing and has a very limited field of view. Both of these can be destroyed, but only with the spade, grenades, or soon to be RPG (http://i.imgur.com/bREFC.png).
LMG: A somewhat different version of the Gatling Gun mentioned above, it is a weapon that can be carried by the player. Obviously, a LMG is a beast of a weapon, so drawbacks are needed. These can include: User must be stationary to fire, possibly with the gun being propped onto a block for support, a limited field of view, the ability for the gun to overheat from excessive firing, and long reload times.
Textures for blocks (maybe): Simply, when creating a map, a template of textures, such as grass, stone, wood, leaves, etc., could be used to create more visually stunning maps. Any texture could go with any color block, which could let the imagination flow freely (orange grass alien world map anybody?) However, considering that it would increase the games requirements, and that people are already creating fine maps without textures, this is not a very important addition.
That's all I got for now, feel free to post your views on my ideas, and perhaps even expand upon them!