Wizarding War Time. Voldemort starts building in like, 1975ish? He gains followers, probably keeps it political for a while. Voldemort... maybe promises more work to keep muggles out of wizarding business. Better infrastructure to the Government, and dealings with Creatures and other Sentient beings. Remember, alot of wizards DON'T believe in Goblin, house elfs, or other creatures rights. They don't believe they should be given special treatment. If the government was working to impliment such things, instead of say... squashing the Goblin rebellions... then Voldemort comes along and promises that Wizarding rights would be protected and considered priority over other creature rights... and wizards are worried about that... then of course they are going to go "Hey, I like Voldemorts idea's on protecting the wizarding community from outsiders" Maybe gets close to being Minister when a radical group of his supporters attack an anti-Voldemort rally type thing. This discredits him and he loses political power. Say around 1980 the radicals grow and become the Death Eaters. Voldie takes charge of them after being booted out of politics post-attack #2 (which came about because Voldie lost the vote). During 1981/2 there are loads of attacks and killings etc. (I'd say a load of kids no longer have grandparents because of this? Probably lost old cousins/uncles/etc.).
Voldie started getting the idea of immortality around 1983 and probably started messing with ways to extend his life. There could be some lulls in violence here as he kept screwing up, basically. Then in like...1985? He tries the horcrux and succeeds (and so later on, he can continue on this route) but is sapped of a lot of energy. He starts researchig into other ways and hears of the Philosopher's Stone through one of his informats in the Order. Maybe the one he used, or tried to, the first time was a failure and thus nearly killed him instead of granting immortality? It takes him 10 years to reach where we are now.
I imagine the DEs continued with random attacks and murders where they deemed possible. They would be countered by the 'original' anti-Voldie group and they could've had battles between themselves, killing many. The threat of his return was always there, methinks, so no one is completely surprised. We need a DE who took the role of leader (maybe the current Minister?) when Voldie vanished. They would continue to gain members over the years as that way of thinking had become popular or something.
It makes sense that the reason that the Order hasn't defeated Voldemort yet would be because that the Order is divided into two disputing factions. Since we might be tying in the Philospher's Stone, could I suggest Nicolas Flamel as the possible leader for the 'rebel faction'? I think it makes sense considering that Dumbledore and Nicolas probably were united in destorying Grindelwald since they are/were partners, but since a lot of time has passed, have grown apart. Also, Flamel is OLDER than Dumbles. JKR says that he's an actual historical person (born 1326), we could use that as influence for the 'other' faction of the Order. In fact, considering his vast history, we could even make the ORDER the rebel faction and Dumbledore as the 'young upstart'.
Flamel forms an anti-Voldie group with a similar take on things as the DE's. Both groups grow and attack each other, killing hundreds on both sides. Dumbles gets sick of the fighting and goes off by himself to form the Order. The Order takes a more non-violent approach, focusing on preventing the attacks and capturing the DE's that initiate them. Flamel could be killed when the incomplete Philosopher's Stone is stolen from him and then his group slowly faded away, leaving the Order to fend for themselves in a non-violent capacity. This issue is now being raised again at recent Order meetings and history looks to repeat itself. The Order wouldn't necessarily continue to gain members as Flamel's group would've been better known, so maybe people thought there was no 'good-side' group to join anymore? Until they become a little more high profile, they're probably struggling in terms of membership.
What if the issue was not eradicating mudbloods and blood traitors, it's about the way magic was used, and for what purposes. Voldemort didn't care, wanted let the Muggle world go on as it is... If his motivation was simply to scramble for as much power as possible. His motivation isn't "Rid the world of mudbloods" it's "BE THE MOST POWERFUL WIZARD IN THE WHOLE WIDE WORLD". However, maybe he just hit on that ancient prejudice as a way to gain followers, because we know people are SHEEP and like to pretend they're better than others. Maybe that was the only way for him to get the support he needed, was to play the blood card. I think that's what he's really all about in canon. So, people need to oppose Voldemort, not because of blood, but because of his methods. Because he's amoral. The other stuff is really just a smoke-screen that matters to the DEs more than it matters to Voldemort. Is that what he should be about in NP? Dunno. Just talking.
Not quite sure how to add that into it, even though the idea intrigues me.
Thank you all for your input!