After the desperate last minute offer of a referendum on AV from the Tories, it looks like that could be the minimum possible now for the Lib Dems - they could prove my suspicions about them wrong - maybe they will get real reform (from chart, you can see why AV is a possibility for big 2 and why the Lib Dems favour STV).
The problem that was Gordon Brown is going, now a Rainbow coalition is possible, however precarious it might seem. A rainbow coalition could have a majority of 32 (Lab 258 + LD 57 + SNP 6 + PC 3 + GRN 1 + SDLP 3 + NI IND 1 + NI ALL 1 + DUP 8 = 338) (CON 306 incl by-election on 27 May). Remember that a 21 majority kept John Major in power for 5 years. Even if the DUP align with the Tories that would still leave a majority of 22 and the DUP are not natural Tory allies.
The problem with stability of the rainbow coalition, will not be the coalition partners who will all be happy to be in government for the first time after effectively being permanently excluded from government before. No, the problem will be the right-wing of the Labour party.
For this coalition to work no more than 21 Labour MPs can abstain and no more than 9 Labour MPs can vote with the Tories, particularly over implementing AV and a referendum on full PR. Controlling these Labour MPs will be difficult, there will have to be iron discipline to make this work, but the alternative is a Tory led government who will gerrymander the boundaries and start cuts straight away. So we could hope that that threat will keep most Labour MPs in line. Neither Labour or any of the other possible rainbow coalition parties will want an election this year, only the Tories could afford another general election so soon.