How about a way to save the layout of a TBM in human-readable format and a way to auto-assemble/disassemble it? This could allow you to make a TBM out of any type of block (I assume that atm it's limited to glass and iron due to other blocks risk being brought along unintentionally) since the layout is explicitly defined, allow people to share their machines, more automation etc. The "construction yard" could be something resembling the buildcraft quarry. You make a cubical frame and the machine is built inside it. Most notably, you could design a machine outside of the game, perhaps aided by some tool with a graphic preview.
Tons of features could be implemented through the save feature, like adding custom paintjobs (think sprites possibly covering more than one block), graphical objects from render files at specific location (creeper hides on mah big evil drill!), maybe even with an animation, maybe even permit fully custom render files. This introduces security concerns, since clients need to download stuff from the server (one could probably embed a virus in a custom render file). You could generate the basic structure locally from block data and ask the user if he/she wants to download the additional graphical content. You could also simply be forced to manually download TBM plans and your client will calculate an md5 hash or something off the file. When you build a certain TBM you send the hash to the server and the server relays it to the other clients. They then proceed to check if they have the hash (and therefore probably also the machine in question) and if so render it and if not notifies the player. Additionally the server could keep a list of "trusted" TBM schematics and if a client lacks a TBM schematic the server offers to send it to the client in question, with the logic that while you shouldn't trust the other clients you may trust the server. Furthermore you may trust specific players, so you could keep a whitelist of players whom you will automatically download TBM schematics from through a P2P connection (P2P to prevent people from shuffling over-sized schematics through the server and thus flood the connection). This will only be preformed on servers which you have whitelisted, since even though you may trust the player, the server may lie about the IP it sends which you make a P2P connection to really being that player.
Also, extending the concept of a frame (and going a bit wild with the ideas), how about the ability to build a frame around a volume, add some engines and attach a controller and then every block within the frame is considered part of the TBM. Basically, you build one around your base and suddenly you can move your entire base. This would at least require the ability to move backward and sideways, but that shouldn't be too difficult (relatively speaking) to implement. Vertical movement may be more tricky, especially moving up, as in how to make it seem a little bit realistic (as in big volcano evil fortresses can't fly just because you put threads on them). One could have a layer of something like pistons (some industrial high-power variant) which pushes the thing up one or more blocks when encountering an obstacle and then the machine continues forward, leaving the pistons stationary. When a piston passes outside the area the machine occupies it is removed and replaced at the front of the machine. Different pistons could provide different lifting height and power, height enabling it to scale steeper slopes and power determining if it can be lifted (implementing the concept of weight, which you already seem to have done).
Of course this is just wild speculation and wishful thinking. I just got so excited when I saw the video.