I'd replace him with Clegg.
Personality politics aside, the election is going to determine whether it's Miliband or Cameron at the helm of the next Government. Cameron has made it clear he's not interested in turning up for that head-to-head, so the next person in command (Clegg) should be invited to defend the Government's record. Inviting anybody else just dilutes the whole point of the 1-to-1 debate, and having Miliband alone is nothing more than an extended Paxman interview that people won't care that much about.
Either way, I hope the broadcasters hold their nerve and proceed as planned. Politicians should never have the power to veto these types of events, and I hope the backlash Cameron faces causes a serious rethink in his stance. You can't act like an ardent support of press freedom, and then attempt to point a gun at their head when arranging election debates. The worst mistake the broadcasters made was trying to work with politicians to make everybody happy. Lay out the terms and invite them. If they don't co-operate, empty chair them and let them suffer the consequences and media spin. End of.