An afterlife version of Anakin is part of the Force itself. Whether it was afterlife Anakin, or the Force itself in a more general sense, there are so many problematic implications either way. To me, that'd be such a massive cheat that would also undermine any sense of free will implied in previous Star Wars.
What this would do is mean that no one's greater destiny can be avoided because "the Force" has bigger plans for everybody. Everything can be controlled or manipulated from another realm. If so, what have we been watching? Luke would've had no possible role other than to ultimately redeem his father if the Force deemed that to be his greater destiny. Everything would be predetermined. The same goes for every other Jedi involved in major events. If they are in peril, it's up to "The Force" as to whether or not to save them in service of their destiny. It'd be such a self-limiting direction to take the storytelling.