At least one NDP leadership hopeful is proposing that the Liberals and NDP join forces in the next election to nominate a single opposition candidate in Conservative ridings. The goal is to maximize the probability of defeating Stephen Harper without actually officially merging the two parties. The downside being that it would decrease the probability of either the Liberals or the NDP winning a majority government, likely resulting in the need for a power sharing agreement in a minority government situation. If the Liberals run 220 candidates instead of 308, it will be more difficult to win 155 seats (70% of Liberals would have to win their seats to win a majority).
So are the Liberals going to diminish their probability of winning a majority just to remove Stephen Harper from power? It's plausible, and will be debated when the Liberals finally get around to beginning their own leadership race. Will it happen? I'm not convinced, but a limited non compete would be the first stop on the road to a merger.