Ok well I suppose it would be possible for it to reach NFL-caliber popularity in the U.S., or NHL-caliber in Canada. Very small, but it is possible.
Soccer however, just watching highlights of cities reactions to wins in World Cup games, especially in South America....tells me it won't ever reach that type of insanity.
Thats the biggest thing that other sports like NFL, football etc. have on MMA, is that they are team games that represent their city, state or country. So yeah i think it would be almost impossible for MMA to reach the levels of a say a world cup, but those are individual events that are only contested once every four years that basically involes the whole world, even non-football fans watch because they are just watching for there country.
But soon a big MMA event will be on live TV almost every week of the year, and if the sport keeps growing like it has into other countrys like china, india etc. who know's? the UFC could be watched by 50, 100 or even 200+ million people every week, and i dont think its that far out of UFC's possiblities to do that.