Interesting! Well the shape of the legs really remind me of the 64's Arachnotron, so that's a good thing. The shading and material of the whole sprite, though, is waaay off when you compare it to the original sprites. That's a given though when you take the style that Doom 3 uses and try to inject it with the other monster sprites.
If you could edit the model's textures to match the Arachnotron a bit better (or just manually do it, but that might be a lot of work), that would help. The shading and details need to be a little more cartoonish. And perhaps you could use the brain part of the Arachnotron as a base to then create the Mastermind's face and body. The fleshy brain model looks way off to me, though again maybe you could edit the texture so that it fits in a tad better.
But at any rate, this is definitely a good start! The scale looks spot on and the overall shape looks like it would work very well