I do see where you're coming from, but I just feel it's more likely that people will start saying 'lol' in everyday speech than using it becoming an official kind of punctuation. I mean, I know people who say 'lol' ironically in real life, and the way I see it it's only a matter of time before it becomes a word. Becoming punctuation... I just think it's more of a stretch. It can't be both, and I think it'll become a word first.