the "key" is figuring out what run filler might be a future key, or semi-key, or semi-semi-semi-key
remember when 4th or 5th appearances of a hot character (sabretooth comes to mind) mattered?
there are so many books that have minor 1st or second appearances in them or deaths... captain america 285 features the death of the patriot, a character that had been around in one form or another for 40 years, was captain america for a while, etc., was featured on SHIELD (sort of) for a year, and it is a $2-3 book raw (no, i don't have a pile of them) ... he's been dead long enough they might bring him back...(of course he would be 100, but I digress...)