His plan would be to let grassroots members of the three parties [Liberals, NDP, Greens] decide in each riding currently held by the Conservatives whether they wanted to hold a joint nomination meeting. If they agree, all parties could run candidates at that meeting and all card-carrying members of the three parties would get a vote, but only the winner would go on to run for a seat in Parliament under the banner with which they ran in the nomination.
Something like this is a lot more realistic than a full blown merger. It at least deserves consideration, since the death of the per-vote subsidy removes some of the incentive for parties to run kamikaze candidates.
Still, I question how many ridings would actually agree to a proposal like this. From a pure game theory perspective, there's no incentive for the Greens to participate since they'd never win a joint nomination. Similarly, I can't see Liberal and NDP members agreeing to an open nomination unless they were both convinced their candidate could win it - that likely limits this to a dozen ridings coast-to-coast. Even if things made it that far down the track, the logistical nightmare of a joint nomination meeting might be too much to overcome.
Cullen himself admits the Liberals and Greens are unlikely to go along with his plan, and he's probably right. But good on him for putting the idea out there for debate. I'm a staunch anti-merger Liberal, but even I concede it's worth at least considering.