I see that we now have an option to have a 16 team playoff. That's cool. Are there any other new customization options added such as adding teams to existing leagues?

One customization added is the ability to have custom bowl/playoff field, which then has it playing at that location vs the home team. Plus, you can have custom field graphics. The real world mod has a base of bowl/playoff fields included.

There is still conference realignment between seasons. You can change the format of leagues and move folks around into different conferences, and in theory remove teams, but there isn't a way to add new teams in.