For me it would:
Have to have a lid
Run on coal but also wood if you want to burn a few logs around it having beers afterwards, so possibly a removable lid so it doesn't get in the way
Enough room to be able to offset the coal and also still put a decent food offering on it
Be easy to pack in the car (so rectangular rather than round)
Easy to carry solution, ergonomic shape and maybe with a bag, heat resistant bag maybe for carrying BACK if the BBQ is still warm
Vents (intake and exhaust)
I would think made of steel but not too thick, I think 1mm would murder it you could put some creases in to to re-enforce it, just depends on how robust it needs to be.
Oh yeah, a garage and a helipad too