There's beena timeline discussing thepossibility of UKIP gaining representation at the party leader's debates prior to the General election? What is the possibility of there being a fifth party leader in addition to Conservatives, Labour, Lib Dems and UKIP?
Next in terms of votes was the BNP but given how Nick Griffin went down on BBC Question time, this might be unlikely.
Next in terms of profile, (Arguably) is the Greens, and personally I'd love to see how Caroline Lucas would do against Brown, Cameron Clegg and Farage.
An alternative is a debate featuring regional party leaders as well.