The model itself is really great, I don't think you need to do anything more to it modelling-wise. But of course, you need realistic mats - perhaps runnels of rust on dully-relective metal, or scorch-marks here and there. Then there's the overall POV and atmosphere. I'd disagree about the sky being perfect - it's boring - you want brooding, fire-tinted clouds, you need some ACTION in there, movement, missiles screaming overhead, explosions, motion blur! At the moment this guy looks like he's taking a stroll on the beach. This could be an exceptional piece of art if you work on it.