I liek what you've done with this, Peter. It definitely shows a years worth of improvement, and is head and shoulders above the heavy tank.
I really like everything about it- the hatch and mg on the front are ace, and that's one of the sharpest tread profiles I've seen. The gun barrel is interesting, making good use of the technic pin joiner.
Here's what I find lacking, though. Technically, it's just one small place- the front corners. I understand why the side plating stops where it does- to avoid the sliver gap that appear between it and the 2x4 wing on top there. I think it would have been worth it to replace that corner wedge with a rectangular equivalent (be that what it may) and extend the side armour all the way to the front. You'd get a sort of "sharp cheese" look, which might, IMO, be better than what you have now.
My aesthetic beef: more colours, man! So far, you have one accent colour with sparing use of greys added to mix things up. Moar accents! Bits of yellow and red work wonders as warning indicators; use a med blue/blue/dk blue stripe somewhere to emphasize the main colourscheme. Of course, I know that you have a specific idea for what NB should look like- maybe my suggestions overstep the boundaries.
Lastly, the semantics. Either call make it bigger or stop calling it an MBT! MBT's are supposed to be very large tanks, and I'm just not getting that vibe with back cut off like that. Don't get me wrong, it looks sharp like that, but I would classify this as medium tank or something.
IMO
-Jacob