So something I've always wondered is if it would be possible for a "stable" form of cancer to eventually metastasize an entire person, and then that person just becomes a walking living tumor but is entirely stable.
I mean, not exactly. Cancer currently kills us because it figured out a version of immortality that will currently outperform your cells.
It is theoretically possible for our cells to mutate in such a way that they have the specific genes that certain jellyfish have, allowing them to become "immortal" by not continuing ad infinatum, and instead having the cell "rebirth" itself like the legendary Phoenix.
Rather than what cancer currently does, which seems to be, a cell forgets how to die and starts replicating its mutated form out of control.