Structural Integrity System Overhaul
The Structural Integrity system needs a complete rework. As is, it seems to rely entirely upon the weight of blocks, some pre-defined multipliers/variables for different materials and the moment (Torque) a system experiences (Such as, 1x1x5 tower with a horizontal beam extending from the top will fracture at the pivot of the system). *(Not sure on last point)*
A better SI system would incorporate separate subsystems dealing with tensile, compressive and shear stresses individually for each material type (Stone/wood/etc). I'm not sure how you would implement this into blocks (I have very little xml/3dmodeling experience).
Perhaps you could use some kind of vector mathematics for a given system; e.g. a 1x1x5 tower would have a compressive stress vector from the top to the base passing through all the blocks and changing with each block (increasing in magnitude in this case). Stone would have a high compressive strength (<1 multiplier to C stress vectors based on irl numbers? Setting stress values of oak wood as the baseline?) and low tensile strength (>1 multiplier to T stress?)
Additionally, alter arch and corner blocks to reflect their real-world properties (reduce tensile stress by channeling force into compressive stress to an anchored point).
This would probably need an immense amount of experimentation, research and time to implement, but would significantly improve the quality of the game. For as the wiki says: "This is what brings the game to life as this is the pure foundation of true medieval engineering."