Every time I look at the Flak cannon, all I can think is, "That would make a great Omega Supreme pair of arms." So, I've decided to make one, only with a newly designed cannon instead of the flak, for a more OS feel.
Before I put too much into the design on this, I wanted to get some critiques and suggestions. I'm trying to keep it fairly close to the original, just with some better proportions and different vehicle modes:

Vehicle modes, partially designed:

I'm thinking wheels instead of tracks, to make the 'tank' mode more versatile. The red chest/abdomen piece opens to allow a single figure to sit inside. May modify it to allow for 2 figures, a driver and a gunner. The cannon will have a piece mated to the back of it to allow a figure to sit. This piece will come off to form the backpack of the robot. I still haven't decided what to use to form the legs. I'd really like some suggestions here, if anyone has any.

Robot mode:

No joints actually created yet; pieces are just suspended in space. He looks really plain right now, so I'm thinking I need to add some kibble, just not sure what yet. Panel lines? Miscellaneous hoses and tubes? A combination of them?

Robot mode, different angle:

Let's you see how flat he is right now, and in need of some detailing, without going overboard. I'm thinking some angled panels on the gray portion of the chest to mirror the angles and bevels at the crotch.

This isn't going to be 1:18; it's going to be 'Joe-scale'. So it will be small compared to true 1:18 Transformers customs, such as the awesome Devastator, Megatron, and Optimus Prime that Keenan did(I think? if I'm crediting wrong, please correct me so I can fix it). All said and done, he'll be about 30", give or take a couple of inches.

Most of the figure will be printed out, but I may cut some of the large flat pieces from sheet styrene. I'm not happy with the head so far, so I haven't added any hoses, the face shield, or detailing because I may scrap it and start over.