The planes are a host to a whole lot of issues, and rather fundamental metaphysical ones.
The chief being that if you treat the afterlife as a place where you just go on adventures as if you were still on the Prime Material Plane, it's not the afterlife. You can get killed on the outer planes, and so it's actually just a reincarnation there. Especially when you are explicitly told that the soul is not immortal there but will be absorbed and so annihilated. (So, not so much an issue with the Elemental Planes.)
The other metaphysical one is that sometimes it gets treated as that which the player characters can not reach while alive can not possibly exist, so if you die in the Outer Planes and do not reappear somewhere in the planes, that's it. (True, it can be hard to prove or disprove something you can not reach, but perhaps others can. Like, more powerful beings, such as gods. Though with players tending to treat gods as hard to kill monsters. . . .)
If they lack the power to leave the Prime Material Plane, that means there is no afterlife and no gods -- a statement I have actually seen. The irony being that there are various monsters -- aberrations -- that are alien to the whole set-up.
There is nothing to prevent a GameLit novelist from having touches of transcendental touch even a RPG-verse -- things from realms beyond your power to reach, and even your ken to fathom. And nothing to prevent the DM except the danger of making the players grouchy. Except, of course, the difficulty of doing it. A wise depiction of metaphysics requires much wisdom.