Not sure what subject to put this under, since it would apply to several. I saw these ideas in the Marketplace and think they would be great additions to Java Minecraft. I would like to see more multi-block mechanical constructs made available in the game. They might not be workable with the current base programming of the game, so this is just a conversation starter of ideas. I am just tired of getting yet another farm item or food for every update.
Catapults and Trebuchets - Hurl destruction upon your opponents or general mayhem.
Wagons - Hook up to a horse or horses to haul several chests/barrels around or use as a mobile base.
Ships - Something better than just a rowboat. Mobile base to sail the seas in. Cannons to use? Could be powered by redstone or furnaces.
Airships - Fly the skies and watch the world unfold below you! Fight in the skies? A mobile base to explore the world in? Could be powered by redstone or furnaces.
Just some ideas of multi-block items that could be custom designed by players to suit their individual needs.
Normal blocks can't move - that's what entities are for, which are an entirely different thing from blocks, or even tile entities (which can also move in a way, but they are fixed to the block grid so smooth movement has to be visually interpolated, so this is best for things like pistons pushing blocks); for example, when sand falls it transforms into a falling sand entity, then back into a block when it lands, likewise, ignited TNT transforms into an entity and can thus freely move (the first that comes to mind when reading about catapults is a TNT cannon). For some of these you could make an entity whose model is made up of blocks, as has been done in mods that add ships that you build out of blocks, which can be transformed into an entity and vice-versa (there is one special block, which is presumably a tile entity, which checks for connected blocks and assembles them into a single entity. The tile entity itself no longer exists when compiled; its data is now stored in the ship entity, like how a minecart with chest or chested horse is a single entity, not an entity + chest tile entity or similar).
While I'm against explicit multi-block machines, I'm all in favor for implicit multi-block machines like existing piston ships or TNT cannons. I would support things like redstone-based block placing and breaking and stronger pistons.