I have wondered that myself. On ONE hand, the $20 is one of the most common notes, and it could be next for THAT reason. On the other hand, the $20 was the LAST note released in the Bird Series (or one of the last, at least), so that shoots a hole through my theory on the first hand.
FYI: Like the Multicoloured Series (1969-75, 1979), the notes with the Prime Ministers are being released in the order that these Prime Ministers held office (Macdonald--$10, Laurier--$5, Borden--$100, King--$50). I think that's neat.
Addendum to my last paragraph--I KNOW that the $100 was actually not released until 1976, in spite of a 1975 engraving, so the order is not QUITE the same. I AM aware of this, so I don't need a bunch of corrections, please.
THE POINT IS.....
I am not certain which note we'll see next between the $20 and the $50.