Personally, I think a lot of people are overlooking the updated and expanded "Group Finder" browser. Being able to browse through groups people are putting together to go run old dungeons, do old achievements, work on legendaries is a pretty cool feature and might help return a little bit of that "we ran ___ with this guy and he was really cool, let's invite him to ___ also."
I like the additions they're putting in to reward pre-formed groups and encourage socializing and making friends in the game again without penalizing players who are trying to get a quick run with LFG.
Also opens up the availability of other players looking to run old dungeons for achievements and such, or just run old raids for the hell of it. You could even try and see if you can find enough cross-realm for "Looking to run ICC 25 with only level 80 characters" or the like.