Currently in the first leatherworker masterbook there is an item called High Allagan Leather. As far as I can tell it is used in 0 recipes. There seem to be similar recipes in every craft's masterbook that don't have as obvious of a name. They've said the 'endgame' recipes are coming in 3.05 alongside esoterics and red scrips. I suspect at a minimum there will be craft materials needed from second or final coil utilized in some of the recipes. Going back and letting you 'augment' coil/towers gear so you could dye it like you can with the artifact armor would also go a long way towards making people want to farm that content again.
A 24 man roulette is a very easy addition to make for esoterics/exp, etc come 3.1. I'm not sure how I feel about doing the same for extreme primal or coil roulette, unless you can go in as a pre-made like with expert/high roulettes or unsynced. Adding wind up versions of every coil boss (i.e. Caduceus, Melusine, Rafflesia) and the extreme primals or some new coil ponies would also help. Give the old coil bosses a TT card for extra motivation. As others have said, the new relic quests will probably require something from old raid content.
Besides that, vanity minions already exist in Syrcus Tower and WoD. Furniture items drop from all extreme primals, towers and turns 1, 2, 5, 9, and 13. Everyone knows bluebird minion drops from the secret chest in turn 3.
Towers gear has always been about glamours so I'm sure people will go back for those eventually, same with primal ponies (especially since they've promised at least Kirin will fly). Nerfing the mechanics in unsynced towers that require more than 8 people will also go a long way towards making it accessible for people to farm.
I think the main issue is that right now everybody is leveling and checking out Heavensward, which is exactly what the dev team wants. They want returning and new players rushing to get into HW. They want current players leveling new jobs and exploring the new areas/content.
I understand your frustration but once we get to 3.1 I'm sure the landscape will have changed enough where there are more reasons to go back to the older content or you'll begin outgearing it enough that you'll be able to go in and do it with less people @ higher ilvls unsynced.