Yeah, difficulty adjustment I can do. I would just start at some difficulty, and announce ahead of time (maybe 6 months ahead) of the new difficulty. People would have to roll over their device keys before then if their PoW wasn't already high enough. But I think @ynniv 's point is the biggest problem. People's hardware is just too variant. Smartphones can't compete with spammer dedicated hardware, so there is no PoW which is easy enough for a smartphone and still too hard for a spammer with dedicated hardware. And my point about the key generation not being the gating factor also has convinced me that this line of thinking is a dead end.