At the point when you have such high level dwellers and gear, and no disaster lasts longer than 10 seconds, even new "elite difficulty" content would be blown apart. Perhaps being able to send a group of dwellers to make it to a special target, but even then, it would be a group of max stat 50's with fat mans. New content of any kind would be nice, but would most likely not be a challenge, and if it were a challenge, only the most eltie of us would be able to play, and so the rest would pitch a fit.
Anything else new, interesting, fun and engrossing would take straight up brand new in depth development. I'm already making a little web tool to show the combination of hair styles and beards available so I, and others, can customize our dwellers. Now, if Bethesda would create some kind of hair, face and clothing editor, that would be fun. Right now there is only 1 male and 1 female face. Different colored clothing, that's all a bunch of Sim City type stuff though. Trophies for creatures killed in the wasteland... just no leader boards that would be hacked within 30 minutes of being established.
There could be quests to look for sets of items, that when combined would create a new weapon, outfit or room.
If you encounter a Super Mutant Master and fail to kill it, it comes for your vault, same for Deathclaws.
Perma-death for dwellers.
Expansion to 400 dwellers, and into the mountain above with Vertibirds to retrieve dwellers that are far out there, but only if you collect all of the vertibird parts from the NGA or an objective 100+ hours out.
In the end though, it still will become a quest to have everyone maxxed out: 644HP level 50 with 10 in each SPECIAL, legendary outfits and weapons for all.
Being able to raid someone else's vault - muahahhaa. So, a special mode where you have a limited amount of points to put into SPECIAL for dwellers to raid each other or just raid a target in the wasteland.
A minimap of distant targets, bases to fortify... but now really we're talking about making a HUGE app that would probably crush most devices, but I can dream.