I'd actually just tweak this a bit and say 2 open air stadium options and 2 retractable roof stadiums (since no one actually builds domes anymore). This combined with a few expansion cities (Las Vegas, Portland, somewhere in North Carolina, Montreal with the option to move back into Olympic Stadium, and maybe one more). The rest would already essentially be in the game in terms of jersey creation (from DD).
I think if they wanted to do it gradually, a realistic option would be to create 2 stadiums and start with being able to move into a new stadium (Oakland and Tampa could move) in a game. Then, in the next years game, add 2 more stadiums and the ability for expansion using the 2 new parks and the current DD jersey and team creation system.
If they do it, it would be nice to be able to play as a team like the Mariners, but still be able to set the home ballpark of both the A's and the Rays (give them a new ballpark without having to play as them).