Maybe just have some sort of visual warning next to the thread title in the regular forum view so people can notice and skip a revivified old thread, kind of like the smileys you can use as post icons or the stars for rating threads?
Something like:
This Really Old Thread Someone Has Decided To Bump Up
piningforthefjords (2007-06-05)
An Ordinary Thread With Maybe Some Cool News From the Future In It
martymcfly (2011-05-25)
Not So Old Thread Where My Ideas, Let Me Show You Them
tinfoilkindlecover (2011-05-01)
An Ordinary Thread With Maybe Some Incoherent Rambling In It
philosopherstoned (2011-05-09)
Ranty Rant Rant About Copyright/Pricing/DRM/Agency/Censorship
darnyoutoheck (2011-03-05)
Of course, there'd have to be some sort of minimum time since last post X years ago calculation so that long-running threads which are regularly posted in such as the What Are You Reading thread in the Reading Recs forum don't get tagged.
Or maybe just exempt sticky posts and/or use the same criteria that the system uses for warning "You are about to reply to a very old thread".