According to The Sports Business Journal, the collective bargaining ongoing between the MLS and the Players Union has seen a demand for free agency and guaranteed contracts for the players.
Right now, very few contracts are guaranteed for the players and then only for the biggest, marquee players. Most contracts are "semi-guaranteed" which means usually that if players haven't been waived by a team prior to the opening of the July transfer window (usually July 15), then their contract for that year is guaranteed and counts against the team's salary cap number. But if a player is waived or released before July 15, their contract is not guaranteed for that year. Multi-year guarantees are virtually unheard of (outside, again, of marquee players or designated players).
But guaranteed contracts probably should exist for all players who remain on the roster past opening day or some other very early date.
Free agency is something a bit more different. In a league where the league itself, rather than the teams, holds the players contracts, free agency would be a radical departure from the current structure. In free agency, the premise would have to be that the clubs would be holding the contracts since players in free agency would be free to entertain offers from multiple clubs in a bidding war.
I am not so sure that MLS is ready, financially, to make that step. It would mean that clubs are going to be doing a little more on their own financially. Some clubs could do that, some couldn't. Even with league revenue sharing, you will develop something of a haves and have nots system. Clearly, I don't think the salary cap is going to go anywhere, so you won't end up with a free for all like most European leagues, but free agency would up the player values in most cases.
An interesting side effect would that some of the quality of play may improve. currently, a player in a last contract year is not really incentivized to play better for his current team if he is in a position that he is sure to get retained. However, if other teams were going to be coming in for his services based upon a good perfromance year, he might be influenced to the point of improving his game to command greater salary from his current team or a new team.
What has been left unsaid, but I am supposing it will be the case, is how much the salary cap is going up. I have been an advocate for a significant bump, plus a living wage minimum salary, i.e. in the $35,000 to $40,000 range for development players and $50,000 to $60,000 for senior squad players.