MMA does have lighter weight classes. The WEC and Bellator, for example, both feature featherweights (145) and bantamweights (135). Some organizations do have flyweights (125), but they are not common.
In terms of adding weight classes, the only possible one I would like to see added to MMA would be a cruiserweight of about 235 pounds. I think 206-265 is too large a gap for the heavyweight division. Other than that, I think things are fine. Boxing is ridiculous with all its weight classes. The more weight classes you have, the less each belt means.
In terms of the UFC adding weight classes, I don't see it happening in the near future. Zuffa owns both the UFC and WEC, so many think a merger is likely at some point. Zuffa, however, seems to be content with their current business model for the time being. It will probably happen at some point, but I wouldn't expect anything for another couple years.