Is there a list of things that will be unattainable after Legion? I only subscribe for a couple of months a year (it's for my own good!) and I like to prioritise these things (MOP and WOD challenge modes were awesome).
Aside from Ahead of the Curve/Cutting Edge for Tomb and Antorus and the rewards
The red infernal mount from Mythic Gul'dan will probably drop to 1% droprate instead of guaranteed one per kill. Not sure about the mount from Sass'zine.
For group activities that will become more difficult to find a group, but actually difficult, I'm not sure. By level 120 mythic 15 should be soloable going by how Legion item lvls have scaled. By the end of this xpac people will be able to solo mythic Nythendra. Hell wouldn't be surprised if a Vengeance DH can now.
At this moment, we're not really sure, like how honor levels will work next xpac. Will they reset and the achievements turn into Feats? dunno. Next expansion is still a ways off. If you want to get rid of long-haul achievements its probably best to grind out 10,000 world quests, fish up all the coins from the Legion dalaran fountain, all class hall questlines completed, 250 legion digsites if that sounds fun to you.