8ball
Decolonise colons!
They will never go with two women. (not that two women couldnt be perfectly good hosts) more that the BBC would fear losing the "blokey 30 going on 13" lads key demographic.
There's Guy Martin for that, plus there's a posh lass for them too and a professional racing driver who could possibly double as a 'Stig' type character.
If you get anyone in other than the original characters you're going to change the dynamic - better to try something a little different rather than try to replicate a formula that was getting tired anyway, and that they probably wouldn't nail right.