Teams owning a controlling interesting in their RSN is nothing new. The Yankees, Mets, Red Sox, Astros, and Orioles all belong to that club, while the Nationals, Angels, Rangers, Padres, and (probably) the Dodgers own parts of theirs. Now, a new team has joined the controlling group, and it's not a team you probably envisioned: the Seattle Mariners.
The team will purchase a controlling interest in Root Sports Northwest, and not a moment too soon, either. The team has an opt-out after the 2015 season that they will now surely exercise, and will be able to negotiate an even better set of rights fees. The Mariners were already bringing in $45 million per year, and with team ownership of Root, will likely be able to push that mark up even further, perhaps to even north of $65 million per year.
For a team like the Mariners that has had a payroll of over $100 million just twice in team history (2007-2008), this will give them the ability to infuse more capital into the team to compete with some of the big guns in the American League. There's also another benefit to this, and that involves the rest of the city of Seattle. If an NHL and/or NBA team relocates to Seattle, the Mariners already have control of the lone RSN in town. A new team would either have to A) start a new one from scratch, or B) partner with Root and the Mariners and end up stuffing even more money in the Mariners' pockets.
The next dominos to fall will likely be the Phillies, Rockies, and Diamondbacks, all three of whom have vastly different broadcast situations. The Phillies are aired on CSN Philadelphia, the Rockies are aired on Root Sports Rocky Mountain, and the Diamondbacks are aired on Fox Arizona. It'll definitely be interesting to see what route each team goes, though I think you can rule out the possibility of a Dodgers/Padres/Astros situation, where a new RSN is built from scratch to accomodate the team. Both Philadelphia and Phoenix have three major sports (excluding the NFL) broadcast on their lone RSN, and creating a new one would likely create a headache. The Rockies are the lone wolf of the group as the jewel of Root Sports Rocky Mountain, and it may end up being more feasible for them to join the Avalanche and Nuggets on Altitude and let their current, college programming-laden affiliate die.