Mad idea about rankings
I think that's kind of looking at it backwards. I don't think the combined ranking would make people more interested in Melee/Teams, but I do think if more people were building cross-divisional bots, such a ranking would be more interesting. =)
I don't think it's the lack of rankings that make people uninterested in Melee/Teams, I think people just like 1v1. And in that case, people will just focus on 1v1 rankings, even if combined rankings are available. I think the best thing you can do to get people interested in Melee/Teams is to work on them yourself!