I could not for the life of me see any possibilities of the Dark Brotherhood, Forsworn and the Greybeards getting involved in the war against the A.D. Yes the Forsworn hates the Thalmor, but remember they also despises Nords with a passion, and rather see them all be liquidated. They already have their little war against them, so I can't possibly see them becoming allies of the Stormcloaks to defeat the A.D
The Graybeards will not get involved as their 100% neutral. They do not believe in using the voice as a weapon. They only use it to worship the gods, not to please man.
As far as the Dark Brotherhood goes... They are their own organization. They assassinated the Emperor without forming an allegation them with the Stormcloaks, so why now? I can't see it happening... unless if the stormcloaks preform the black sacrament, but even then I don't see the Stormcloaks doing such thing as that.