If Old Major was correct in his prophecy, Napoleon or whoever follows him will be overthrown and a true revolution will place the animals in control of their own fate.
If Old Major was wrong and the revolutions that take place are all doomed to failure due to the nature and weakness of "people", then the animals will continue to toil as an under-represented proletariat, relegated to hard work and low pay.
The novella ends with the animals peering into the window of the farmhouse, unable to clearly distinguish between the men and the pigs--a picture, of course, a final image of the completed circle. Man owned farm and mistreated animals, animals revolted and took over farm, power-hungry pigs took over farm and mistreated animals, now they have become the men and presumably, as mentioned above, the animals will revolt again, eventually. The animated movie shows the animals attacking the men after seeing this completed transformation through the farmhouse window, so that director saw a quick revolution. I tend to think of it as being slower to come; however, there is little doubt that Orwell intended that to be the eventual outcome. He gave no indication that it would be anything but a continual cycle of oppression and revolution, given man's hunger for power.
Well, if you follow Orwell's idea that "Power corrupts, and absolute power corrupts absolutely" you would have to guess that eventually there would be another revolution. The animals would get so tired of being mistreated that they would rise up in the hopes that their new leader would truly treat them all equally and bring relieve to their oppression. However, just like the pigs, after the new group got into power they would most likely let the power go to their heads and they would become just as corrupt as the pigs before them...as the pigs did like the humans before them.
My guess is that the future will look like the past. The pigs will continue to rule over the farm for the foreseeable future. They will become more and more tyrannical. Sometime in the future, there might arise another animal like Old Major. At that point, perhaps there will be another revolution. But it won't make any difference. There is no real reason to believe that a revolution led by the dogs or the horses or anyone else will be better than the one led by the pigs.
I think that much of this is going to depend on the point of view of the reader. The bleakness of the ending cannot be denied. The animals' hope of a revolution whereby their voices are fully authenticated and respected is dashed, where no difference between Napoleon and the humans is discernible. From this point, Orwell seems to indicate that there is little hope. There is no change for the lives of the animals of the farm. The consolidation of power for the pigs and dogs is complete. The ending does reveal little hope for the future. I think that if the reader possesses a cynical attitude, perhaps one would feel that there would be no level of change and that the consolidation of power would be complete. However, at some point, change can present itself in manners that cannot be envisioned. For example, the change called for by citizens in Egypt and even Libya could not have been seen as recently as a decade ago. Yet, the presence of social networking as well as globalization to allow young people to understand the need for change helped to bring about a shift in power. I think that one could see a similar situation happening on the farm, though the exact elements are not presented to us at the end of the work.