Necromancer (silver); existing, included for comparison
Answered more than 60 days later with score of 5 or more. This badge can be awarded multiple times.
(Removed "a question" from description.)
Revival (bronze); name credits to Gamecat
Answered more than 30 days later with earliest answer scoring 2 or more. This badge can be awarded multiple times.
Reanimator (gold); name credits to Gamecat
Answered more than 90 days later scoring 4 or more with earliest answer scoring 2 or more. This badge can be awarded multiple times.
Necromancer retains its current (slightly different yet still useful) meaning.
A query shows 473 retroactive Reanimators; compare to 1,335 Great Answer and 54 Reversal badges.
Both Revival and Reanimator require an answer which would remove the question from the unanswered list — this feels like the main goal of the badge request. (However, neither yet deals with accepted answers, so answers may qualify for questions that did not appear on the unanswered list.)
All Reanimators also earn a Revival badge (similar to Nice→Good→Great), but also encourage answering older questions (90 instead of 30 days) and require a "slightly better" answer (itself scoring 4). I arrived at four by playing with the query; this may need tweaking, but answers to older questions often don't attract much attention.
Both Revival and Reanimator have a race condition: answering an "unanswered" question could bring attention to it and thus upvotes for older answers — resulting in disqualification to the newer answer. Similarly, you could later be disqualified by earlier answers reaching a score of 2, which won't remove the badge but may make it much harder to collect multiples.
All three badges should apply to CW answers. Only 13 of the 473 Reanimators are CW.