There might not be much to add, since a lot of good advice and perspectives have already been shared. But I will say that a character's motivation should in no way be determined by what is likely possible or impossible for the outcome.
The Gundarakite rebels overall goal is to attain independence and liberty. The fact that Strahd will always remain in power, does not impede in the slightest on the Gundarakite Rebellion role play. As others have stated, it is the journey.
My tanarukk's goals are also highly ambitious, as she dreams that any day now Gruumsh will deliver an army of orcs and tanarukk, and she assumes her purpose in Ravenloft is to scout and prepare the way for the impending war. That, or her destiny is to give birth to a tribe and army of tanarukk who will carve our a kingdom of their own in a world that has so-far denied orcs or tanarukk any place.
Your character's overall motives or purpose can be large-scale, but within that there is potential for more mundane motives and unforeseen adventures.
I also want to challenge a notion I've seen crop up now and then, that evil wins and good loses. Anyone who thinks that any of the darklords in Ravenloft are "winners," isn't really thinking it through. All of them are cursed, and serve as examples of how evil loses. Traditionally, gothic theater was a morality tale, in which evil acts may for a while earn someone political or material power, but always at the expense of their souls and true happiness. In Ravenloft, in terms of morality as being the benchmark for "winning or losing," any character who manages to avoid becoming jaded, selling out, losing their soul, losing their way, falling from grace, etc. etc., is a character that wins. Even a character that dies, may be a "winner" in the sense of morality.