Just thinking out loud:
If teams stayed in the same league, even after a champion is crown. And the only free agencies/players available are from your league. You would most likely see less 40-60 year old players in this game. And in a sense players would retire, at a "normal" age. Not sure, but more training of younger player's would probably go on. And this is how the top team, the aka champion team, would not stay atop "forever"
Another thing, im not sure how many active teams there really are, but you would have less if any bot teams. This part has nothing to do with the new teams in this league, but new teams are kinda scary because you DON'T know if their be active or not. This wouldnt change things either way, and really has nothing to do with anything.
If theres an odd # of active teams, add an extra team or two to leagues, OR have 20 teams per league. (10 on each side, with only 4 teams making playoffs) i think it would definitely keep the playoff race interesting up until the final day. Most seasons than not anyway.
*More realistic BB, then it is right now*