When I've had a think about it before I've considered 2 ideas.

Either returning like the robogames pilot with Grant from Mythbusters- as a reasonably dry-but-interesting-to-hardcore-fans kinda thing (I would enjoy this but I don't think the general population would particularly).

Or something similar to what Craig said above. I liked the idea of some different "arenas"- sort of "underground" stuff, the sort of things that were on the old robot wars games- old factories, scrap yards, maybe inside an old house or caravan (with some furniture around to produce some destruction). Perhaps we could follow a few teams for a few weeks (scrapheap challenge style) in different heats in these different settings, maybe ending up in a studio arena for the finals. We might even get to see some adaptation for the different settings (and be able to shoehorn in some light engineering info like "bigger tyres be good for off road" ect

Just my very casual thoughts