How many weeks do you have available for the playoffs? I've assumed 3 weeks, and up to 3 games per week. Teams in the promotion playoffs would not have scrimmages, just like if they are still in the cup. They would play 2 or 3 games per week based on whether they sweep the first two games of three game series.
For a losers bracket, I don't mean that it would be a double elimination format where teams with a loss have to play more games or series, but simply play against similar placed teams:
S1 G1 v B4 (Series 1, Great 8 1 vs. Big 8 4, home court advantage listed first_
S2 B2 v G3
S3 G2 v B3
S4 B1 v G4
S5 W1 v W2 (Winner series 1 vs Winner series 2)
S6 W3 v W4
S7 L1 v L2
S8 L3 v L4
S9 W5 v W6 (home court advantage decided based on higher regular season place, better record, or head-to-head, etc.)
S10 L5 v L6
S11 W7 v W8
S12 L7 v L8 (or you might sliminate S12 or reduce a single game plus a scrimmage)
There is assumed to be some economic disadvantages to losing vs. winning. Playing in the championship series should sell out (can we charge more for playoff tickets?), while playing for 7th place after losing two series should be rather subdued.
By combining the league record and the playoff record as far as determining extra promotion opportunities to replace bots, you give credit for the league performance, but an opportunity for teams to move up. But even if teans lose in the first round, there will be a reason for them to continue to try to win.
There might be a different format for the bottom 8, that has fewer games, but continues the competition over 3 weeks. For example, you could have the teams in one conference play the 4 teams in the other conference, completing the home and home series, and rank teams for relegation based on the 14 games between the bottom 8.
Again there is value in the league record, since 10 games would carry over, but teams would have a chance to move up and avoid relegation against relatively weak competition. The 5th place teams will have lower risk of relegation because they will have a game or two advantage, and the 8th place teams might have to go 4-0. But even if they are 0-4 and confirm their relegation status it is better than sitting idle.
So the top 8 would have between 6 and 9 playoff games, and the bottom 8 would have 4 classification games, plus 3 scrimmage openings.
I understand that the teams in the upper divisions are due to teams going inactive. If BB is like HT, then around 1/2 of teams will sign up and do little more. There might be another 20% in the next season, and then 5 to 10% in each following season.
If you put teams strictly in the bottom division (instead of the bottom 2 like HT), then all teams that get to the 2nd bottom division will be more likely to continue playing. Players who like the game will tend to do better, those who do better will tend to like the game more. And you can add new teams at about the same time. If you start at IV.1 at the start of the season, and never look back until you get to IV.64 teams will all have about the same starting date. Personally, I'd start them at 0-0. It might seem a bit odd to have a 3 game league, but if everyone is in the same boat, it was fair, and you didn't tell people to wait for 5 weeks until the new season.
If you do get to IV.64, then start looking for the leagues with the most bots, and replace all the bots at once (or in as short a period as possible). There can be some league that have 16 bots, and others that have just a few.