I have an idea that is sort of based on previous suggestions: The Top Gear Ashes.
Basically, a cheap car challenge but with 2 teams: 1 team being the TG boys, the other team being TGA's presenters. Each respective team's car should be representative of their country and totally awful - perhaps a Rover SD1 vs VN Commodore, or maybe a clapped out mid 90s Ford Mondeo (I know it's not made in the UK, but it's still a very British car) vs ex-taxi AU Falcon. Use it as an excuse to send Jezza, Hamster and Captain Slow to Australia, it's been a long time coming. The winning team will get to set the losing team's car on fire. Put some of the ashes into an urn and there you go. The Top Gear Ashes.
Would be a bit of fun for Series 12 of TG and Series 2 of TGA.