[asid61]

First of all, that looks really nice, great render! Your modules look beefy even if they are a little tall, and your 3x3 siderails look strong as well.

Some possible changes you could make:
-4 cim + 4 minicim, 1 of each on each wheel. This lets you have omnidirectional motion. As you have it right now you can move in only 2 axes, and 4/4 gives you even slightly more power than 6 cims. That being said, I really like the form factor of your CIM gearbox.
-Are those HTD profile or trapezoidal profile timing belt pulleys (looks like the latter)? HTD and GT2/GT3 are much better due to the greatly increased working load if you are using trapezoidal.
-I can't open your assembly that I downloaded from Grabcad. If you want other people to be able to look at your full assembly, you need to use File -> Pack and Go and upload that. That will put all the parts and assembly into a single folder, and update all references. Or, upload a step file (but the former is easier for SW users to deal with). Right now I can only view the file in GrabCAD's viewer.
-Good use of extended inner ring bearings for separating the shaft collar from the outer race of the bearing.
-You may not need a 2-stage VP for rotation. You get a speed of 220rpm for turning each module with just a 10:1 x 84:14 reduction, but it's your call what suits you of course. Changing would result in a 7A draw on the BAG motor normally.
-Double shielded or double sealed bearings are much better than open bearings for your shaft support, simply because stuff doesn't get into to them as easily. Look into using 6800 series metric bearings as well; they can be super handy due to their decreased OD (and resulting lower weight). I don't know if you need to switch for sure though, due to the low turning speed and orientation.
-Why the spacer in between the caster box and the 84t gear?
-Using long screws to clamp on a tube to connect the corners will not be very strong. You should stick an insert inside there to prevent crushing (plastic is ok)
-Your free speed looks like 19fps, is that correct?
-Nice attention to detail relieving the corners for the VPs! I really like the amount of thought you've put into this. The attention to small details really is amazing.
-You may or may not want a bellypan, as the sheer (shear) strength of the frame rails might be enough.