I would like to suggest the cast of 'Garth Marenghi's Darkplace' (Matthew Holness, Matt Berry and Richard Ayaode) as the ideal guest riffers.
The improvised DVD commentary to 'Garth Marenghi's Darkplace', where they remain in character, is pretty spectacular at times. Plus all three are writers.
Any chance of inviting one, or all, for a bit of riffing?
Mark.