This is already totally possible in BB as it's currently released and has been for a very long time.

here's how: Slowing down to a hold, or "rit"........ I've used this multiple times. Those tempo changes often occur over a very limited section of the song at the very end. In my experience, it happens over a 2 measure (not 6 to 8) range. To go from lets say 120 bpm to a natural sounding rit/hold.... I do the following. I change the last 2 measures to TWO beats per measure and add 2 new measures also at 2 beats per measure. Then I will slow the 4 measures by a certain amount with each one slower then the preceding one. for example from 120 to 103 to 94 to 81 to 72/hold. I listen to the result and adjust the numbers as needed. The reason for adding the 2 measures is so that the slow down doesn't happen in noticeable steps but is more gradual and natural feeling. This is a song where I used this very technique at the end. Dust On The Floor

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