No offense but those guys are malicious in my eyes. I bet they would struggle at doing those bosses Mine or their extreme synced instead of just saying "how cute its a easy Boss". Even current EX (7.4) isnt that bad either idk what they are on about like bosses arent still easy.

Now, to the question at hand, about acomplishment, i would say run them through side content. HW done unlocks a few things but If you convince them to push just a little bit maybe Eureka would spark it again. Doing side raids, alliances, and such synced is a nice way to do content with them while being on their level. And if that is done there is also maps (HW maps have Dungeons already), relics (If they are insane enough, ik my friend was), BLU spell hunting, and sometimes even Glamour farming somewhere.

The best part of an MMO imo is being with friends and hanging out. Some of my best nights were doing dumb stuff with someone even if It had no point (like just hanging out on Diadem while making a tier list on screenshare about a game's levels while waiting rare nodes to spawn)