Much more expensive than just adding a few developmental players to the roster. If they did, someday, do this (maybe not a bad idea, I'm not sure), I'd want them to create their own teams, not join up with existing ones. Pro/Rel will never happen in the U.S., at least not this way. In one direction, owners would never tolerate their team being relegated based on on-field performance (I think this will always be a problem, unless the minor leagues are nearly as popular as the majors). In the other direction, most teams from a lower division don't have the financial ability to survive in the higher division in the first place, or they'd be there already. Now this idea I have been thinking of for a while. Instead of regular pro/rel, each league could require teams to play successfully and prove themselves (in terms of median attendance etc., not just on-field performance) in the next lower league before applying to join at a given level. MLS would only consider teams from USL 1 for expansion, for instance. It could go as far as the PDL. I think this would result in a stampede of new PDL teams being formed. Some of which would then move up to strengthen the higher divisions. In the other direction, there should be some attempt to encourage teams to drop down to a lower division when they're struggling rather than going belly up (or moving to another city).
There were, but some (including 2 in California) were forced to drop to PDL because they couldn't afford their only other choice, going up to USL 1. Adding 3 more teams in California, one in Utah, one in Colorado, etc., would very significantly shore this up and I'm sure there would again be a western division of USL 2 as a result (ie, the Gauchos etc. would come back), which would in turn give these hypothetical MLS-affiliated USL 2 teams enough competition to make it worthwhile.
Didn't MLS already venture into the world of the USL a few years back with the highly unsuccessful P-40 team ?