that comes to mind is Lady Coldheart (?). The reanimated corpse of Caitlin Stark. That one is totally pointless.
~Jeordam
It hasn't gone anywhere, but that is mostly because Martin hardly wrote about it for the last ten years.
I think it is supposed to serve at least two purposes.
1. To show coming back from the dead has it's price, and you are not exactly who you were when you died. I guess the changes in book Jon will be subtle, but readers will be motivated to look for them because of Stoneheart. I don't see much change in show Jon but that might be a different matter. Or I'm just speculating.
- Stir up conflict between Brienne and Jamie.