Option 1: Realignment that maintain the National and American Leagues
West: Arizona, Los Angeles (NL), San Franscico, San Diego
Central: Chicago (NL), Colorado, Milwaukee, St. Louis
South: Atlanta, Cincinnati, Miami, Nashville
East: New York (NL), Philidelphia, Pittsburgh, Washington
West: Seattle, Portland, Los Angeles (AL), Las Vegas
Central: Minnesota, Chicago (AL), Detroit, Toronto
South: Texas, Houston, Kansas City, Tampa Bay
East: New York (AL), Boston, Baltimore, Cleveland
Remember what I said before, I'm not saying this is what I would do, but it feels like probably the best way to keep the leagues in tact when adding two new teams if they decide to break up divisions.
There is always geographical wonkiness with divisions, so unless they specifcally look at cities to expand into that "fit" within the needs they would have for divisions, some of these will feel a bit odd.
I ended up adding a "south" division to both leagues as that felt like the easiest way to group teams geographically based on how the pieces were falling, and both leagues received one expansion team to their mix. Keeping as many of the historic rivalries was a priotiy for me as well, which can been seen in how some of the divisions end up shaking out.
I wouldn't hate this if they decided to do this, but like I said earlier, they've really begun to open the doors for some wonky maneuvering, which is why I see option of geographical divisions, ignoring the current National and American Leagues, as a very real possibility