This YouTube from PBS Space Time presents some interesting timelines on how the universe will end depending upon different models.
This presents some interesting challenges as an author writing this far into the future.
The first problem has to do with the universe is decaying away to nothing, so how do you build a society on matter that is fading away to nothing? The presenter is assuming that humanity has some material technology present, but do they? Figuring out how to build a space society in a decaying universe is a true challenge.
The second problem is how long are humans living by this point in time? Have we conquered longevity to the point a single human life span is billions of years?
The third problem is how many humans are there? As species live longer and longer there is fewer and fewer of them. So one could imagine there would be few humans living at this point.
The fourth problem: Are there any other intelligent species (created or otherwise)?
The fifth problem is the technology advanced enough to travel into parallel universes? Could mankind be extracting materials and energies from these universes to help deal with issues in our current advance decayed universe? The presenter suggests that parallel universes may occur as part of the decay which gives hope for humanity, but what happens if these parallel universes operate with human killing physics?
These and many more questions happen before you even think of a plot.