How crazy are you looking to get? Way back in 2010, we build a "
demo" bot with 15" mechanum wheels. It was quite the fun project as we had to make everything custom. It even has a torsion beam suspension! It's great in the sense that it's a great all purpose robot, and can climb almost anything (barring you have enough power). Downsides are its expensive, heavy, and requires quite a bit of juice!
TLDR; Big wheels are cool, but expensive.