Li Shenron
Legend
If Epic is going to mean simply high-level like in 4e, then it can be in the PHB, but only as long as it doesn't detract too much space from the lower levels.
I however would prefer to have an Epic that is very different from high-level, i.e. not just "scaled up". An Epic that is unbalanced, in the sense that epic characters are unbalanced compared to the rest of the world, but balance between epic characters is somewhat preserved (I think 3e didn't do a good job with this). If this would be the case, it should definitely belong to a separate manual.
I however would prefer to have an Epic that is very different from high-level, i.e. not just "scaled up". An Epic that is unbalanced, in the sense that epic characters are unbalanced compared to the rest of the world, but balance between epic characters is somewhat preserved (I think 3e didn't do a good job with this). If this would be the case, it should definitely belong to a separate manual.