Please don't put us with the Liberals, again please. 
An interesting coalition, that one might see in the future is that of a BQ+Green+NDP one.
If this merger were to happen, they would have ~35% of the vote. Probably not enough to beat the Tories.
The Bloc, while left-leaning, gets votes based not on ideology but on its support for sovereignty. Most of its voters are substantially to the right of the party, which is why its major competition outside of Anglophone areas is the Conservatives. If it were to attempt to play a role in national politics(which by implication would require dropping sovereignty) it would likely lose most of its voters either to the Conservatives or to a rightwing alternative which is what happened to the PQ when they took that track last time around.
So the Bloc will take part in national cabinet when hell freezes over, and in one with the Greens and NDP quite a bit thereafter. The Bloc is not into suicide, they do want to collect their Canadian pensions after all.
