I will preface this by saying two things. First, I do not know if this idea has been proposed before on the mod idea subforum; if so, indicate likewise or ignore this thread. Second, for mod authors looking to make this mod idea into a reality, understand that I have very limited coding knowledge, and therefore some elements of this mod idea may be proposed in ways that could be actualized more effectively. Otherwise I would have made the mod myself by now.
With that said, my mod idea is this: the player creates a large multi-block steampunk locomotive combat tank as an ultimate weapon of destruction and warfare. It will differ from mods with tanks that I am aware of, such as Flan's military mod in that the tank is not just one entity, but a composite, complex moving multi-block structure, and each block that is a part of the tank stores its own damage data and will fall off or disappear when it takes too much damage. Simple in concept but possibly difficult to program, the idea is entirely inspired by the game Steamlands on Nitrome.com.
In keeping with the theme of essentially every minecraft mod like this, the player starts out with blocks like wood in the beginning of the game to create the tank's chassis, which is the base structure of the tank. The player will then work his way up to more durable tank material such as stone and iron and obsidian as he acquires more resources. Weapons for the tank can be crafted with resources to make weapon 'blocks' that connect to chassis blocks. Tank weapons fall into the category of either ranged weapons (guns, cannons), or melee weapons (rams, saws and the like). When a weapon is placed on the chassis of the tank, the weapon assimilates with the tank's structure as a whole. The player will also need to worry about the tank's power needs; I can easily see power blocks being made for the tank that supply energy for a tank's guns and locomotive systems in the form of standard RE power, or whatever system is most logical.
In addition, due to the nature of this hypothetical mod and what it would become if made into an actual mod, it has large potential for multiplayer use. Therefore, it goes without saying that multiple tanks can be created and attack each other just as in Steamlands. In conclusion, if you think this idea is worthwhile and would be enjoyable to play, give your opinion.
TL;DR - this mod request is basically the game Steamlands on Nitrome.com, minus the 2d side scrolling.