A "bubble" is unrealistic, even a series of bubbles, because its not possible to have all players living and playing on a single site, unless you had them all in one training facility and played games on all the bakc fields, and would there be room for players to live?
What you might be able to do is have all games played in LA, Chicago and NYC, with each team quarantined in a separate hotel in those cities and travel on buses driven by drivers who are quarantined as well. You'd also need all stadium workers and broadcast teams (those required on site, not the local TV/radio crews who could remain in their home cities) to remain quarantined as well.
With each team playing roughly 6 games a week, you would need each of the 6 stadiums to be able to host 15 games per week, which you could achieve with staggering of games. You'd probably be looking at 3 games a day Saturday and Sunday with ~9 games from Monday to Friday.
Lets say Saturday and Sunday games were scheduled 12pm , 5pm , 10pm. With weekday games at 1pm and 7pm. There would be no possibility of Double Headers or Extra Innings.
You would need 6 individual locker rooms available in each stadium that would have to be deep cleaned after each team leaves. Maybe you could get away with 4, with teams from games 1 and 3 using the same ones. There is plenty of space on the concourses so its not impossible.
If this seems too difficult you could add a third Major League Stadium for each region. San Diego, Milwaukee and Philadelphia are all within driving distance, you could maintain the hotel/bus/stadium quarantine.
If this was the plan, the league and players would need to come to agreement quickly, have everyone go into Quarantine for 10 days immediately and try for opening day 2.0 next weekend/start of the following week. That would give the league time to build additional locker rooms.
There are tonnes of challenges with this, theres no way they could get agreement on it for a start, I think a more realistic option is an eventual cancellation of the season.