I'm saying this with no inside information, just taking a shot in the dark. But if it's true that only one school is going to be added, it's definitely going to be an addition to the National Division (why announce that Cuyahoga Falls was moving down to the open spot in the American, if not?).
I'm going to predict that Strongsville is going to break away from its alliance with Brunswick (and Medina), and will get the spot. The move would lock up the Route 82 corridor of Strongsville/North Royalton/Brecksville/Nordonia/Twinsburg, and Strongsville has relatively quick access to Hudson and Stow via the turnpike/Route 8 (the league would pretty much be a "T" along 82 and 8). Wadsworth is an outlier even if it were Solon, and Strongsville is a little bit closer for them.
On top of that, Strongsville, while still bigger than Solon, is at least in the same student range now as the other SL schools and competitively, it competes more on that level, where Solon is in the GCC tier. Being a Strongsville grad, I know the district has, in the past, made it a point to try to compete at the top level possible (they used some pull in the NOC to stay in the Valley when by the bylaws of the league, should've went down). And while I'm now way outside the thought process of what the administrators are thinking, I have to believe, that they know they can't continue to compete at that level, especially since it looks like Brunswick is righting the ship (a lot of talent in the younger levels) and Medina is finally starting to build programs that match the "sleeping giant" talk that has been with them for the past 20 years.
From what I've gathered from conversations, Strongsville/Brunswick/Medina are trying to stay together, starting with the SWC and then the SL (adding a third tier). But if it comes down to one spot, I think it's going to be every school for itself, and Strongsville actually makes the most sense.
And selfishly, since I'm now vested into the Medina program, if the district still has its eyes set on the Suburban, it's going to set back everything that the football/basketball coaches have been building. Out of all the top athletes in those sports that went to MS at Root and Claggett the past two years, Medina High School has retained all outside of one. They have also added at least one St. Francis Xavier kid this year, who probably could've went to a good parochial basketball program. If they end up in the Suburban, I can guarantee that they will again start losing kids to both the Cleveland and Akron parochials, and good luck getting any good athlete from St. Francis Xavier to come over to Medina HS.
It sucks for scheduling, but a league of Medina, Brunswick, Solon, Mentor and Euclid (hopefully would stay) is enough to keep a football schedule at least manageable. The other sports, they can work that out.