What we need is a thc producing giant redwood that buds every year, you could pass it down trough generations. Like this here is my great great great +++++grandfathers 800 year old cannabis tree. It started spreading and it's weed like genes have made it take over the entire forest and killed every other plant here. A forest fire here would get people stoned on the moon.
If you are looking to get into Cannabis breeding and are interested in plants that can spread and create their own 'natural' wild crops, you don't need to go so far. If you live in an area where the winters are short or relatively warm (ie. near the equator), many varieties will grow wild and do! I mean, the shit people grow indoors wasn't always there, and they call it 'weed' for a reason.
Also this new batch of auto-flowering plants have great promise even in more northern climes. Depending on your grow-season's length, even in southern Canada or the Northern USA, you can get at least 3 generations per season, even more further south. And suppose you undertook a back-yard genetics experiment, breeding the hardiest auto-flowerer you can find with a cannibis indica variety that is both extremely hardy and resistant to cold temperatures. It might be possible, after several back-crosses and in-breedings (you would kind of have to know what you were doing), to plant a group of seeds or full grown females and males that haven't yet fully flowered, and let those plants pollinate themselves and go several generations before winter. If you've done your breeding right you might even be able to breed a plant hardy enough to survive through the winter and continue the cycle again the next year and the year after that.
You would have to keep your breeding records so you would know when to go and weed out some excess males right before the females flowered if you planned on harvesting bud without seeds. If you moved away though, the plants should be able to keep on going without you, and maybe hungry deers would eat the flowers and carry your seeds all over the state!
thats kind of a pipe dream too, but I think its a little more plausible than many of the other suggestions. Someday I might try this out, If we can over-grow the establishment there will be a better chance of legalization. Make a strain that grows wild all over America and produces smokeable bud (although there would be a potential reduction in potency after several generations) multiple times per year and it will be much harder to keep it illegal... I hope, anyway.