I think there is still a need of incentives to answer many more old, unanswered questions. Current badges don't help much: a bronze badge doesn't matter; getting 5 votes on a tumbleweed tag/question is very unlikely, however good your answer might be.
So, I propose: Persephone (gold badge, can be awarded multiple times): Posted 25 good answers to 90+ days old unanswered questions.
It's a mix of Revival, Necromancer and Socratic, with many details from Illuminator. In particular, Socratic is awarded for doing many times something good which didn't necessarily score much. The difficulty should be about the same, I think, despite the numbers being different. The name is also taken from Greek history, this time of a woman; alternatives could be Flamelicus, Zombie Army, Lich King or the original Reanimator.
(Other past suggestion were Highlander, not precisely defined; and The Long-Haul Trucker: Answered n Tumbleweeded questions with at least one upvote.)
See the query and play a bit with the numbers. I get only 38 "winners" on SO, with this necromancer earning it 6 times. However, there are 504 users with 10 or more: with some encouragement they could go the extra mile and clean up a bunch of old questions.
Maybe the badge would scale better and achieve wider impact if it required more answers but less upvoting? People would answer questions worrying about quality rather than upvoting-reactivity of the asker. This alternate version requires only a 0 score where the question had no previous answer: currently 27 winners on SO with the same truly unsung hero at 11 times (!). An additional requirement for "maintained a positive answer record" could be added, similar to Socratic, but trying to post 50 garbage answers probably gets you banned way earlier than you get the badge.
Update: I did some more runs of the query on multiple sites and it seems to be pretty general. Some sites would have very few awarded, but they would whatever the settings: perhaps just few unanswered questions, or nobody working as unsung hero/necromancer yet. On the many sites where a certain number of users would be matched, the criteria seem fair: wheter the min score for already-answered questions is 5 or 1, or min age is 90/45/10 days, the number of answers qualifying of course varies, but the pattern is always the same. I mean, we would not be unfairly excluding people who only focus on questions X days old or already-answered etc.; altruists hunting for unanswered/old questions to address seem to do so across the board. :)
But then, I'd use double checking of the query.