I keep coming back to them.
Angels and Time Travel.
Weeping Angels from Doctor Who...no, but that might be something to consider further down the track.
But Doctor Who (the current series) has certainly informed some of the concepts I'm considering.
I'm thinking of a game with fixed immutable events in the timestream.
Our angels are more akin to troubleshooters from the game Paranoia.
Something has twisted the timestream to give a huge advantage to a power that opposes our troubleshooters/angels/immortals/bodhisattvas.
A segment of the timestream is ripped out so they can go in and fix it. They have 48 hours, but they can't cross their own timestream. Any segment of the 48 hours that is used becomes a segment they can never enter again. Every action they undertake in the timestream becomes an immutable fact within the timestream...but they can change events before or after those facts to alter the specific meaning of the events they have already undertaken.
I'm seeing a timeline, lots of index cards scattered along the timeline, and pieces of string that link one event to the next...often having a single events cascade across multiple future events and multiple past events leading into a single future event.
Plenty of conspiracy theory fodder. Anarchy...but if it doesn't work out in the end, that chunk of time will be wiped out of existence...the angels along with it.