World ranking won't be consistent enough to formulate a ranking among teams. Maybe we'll rank the regions, but to have a consistent elo system between regions would require SO much more international play, which Riot might not do(see league)
Especially with how volatile the scene now(Version1, KRU, Sharks might not qualify) it doesn't make sense for there to be a ranking with 20 teams that don't play at every international event that only happens once every quarter.