Here's one spitball variation, though I guess it'd be a little more like to a Behest...
There are various little rifts throughout any zone (level of pre-information TBD). Upon reaching them, or after a small process of assembling related clues in the area, you can lift your clear Guildleve glass (those stain-glass plates, though this one normally without an image) and it fills with a particular, stylized image (in old 1.x fashion alike to an exceedingly large Tarot deck), and some hints as to what would be required of you. If you then focus your Echo and peer through the glass, you are transported to a particular time in that place, and enter automatically into a short quest in an instanced space. Anyone in your party and in the zone can choose to join and be transported there automatically. No overall timer -- only that of very particular events that thematically need it -- and you can mostly exit and reenter as you please, including to do DF activities.
:: When completed, you and your party can exit the Echo through an interactable rift, to the position of that rift in the "uninstanced" world, or exit via the UI to return to their previous positions from which they entered.
Alternatively, Guildleves as... Bounties, of sorts. No timer or the like, as not to punish queuing or swapping over to FATEs. Instead, you simply have additional Key Items by which to spawn particular enemies, and certain gatherables and nodes are uniquely visible to you.
We could say the same for anything. "Sometimes you just want to gain a level without feeling like you've started on a leveling progression you may not have time to complete." Etc., etc.
Chains don't give outright compounding experience. They're just related FATEs that sometimes unlock a FATE that's meant to be more challenging and thus more rewarding.
My point was just I feel the world shouldn't just be... stagnant... outside of FATEs. Rather, the FATEs themselves should just be proof of the zone's motion, calling attention to any of its many strands when they give opportunity for bigger spectacles or larger-scale combat.