Kanzenshuu

A spin-off series revolving U6 Saiyans would be interesting

Well if they end up being more powerful than when Goku or Vegeta first turned SS, it could be explained that these U6 Saiyans have more latent potential than the U7 ones because of the way they evolved differently.

What they should do is reboot GT as soon as Super is done. That'll give them another chance to redeem themselves from the original and gives them another series to continue Dragon Ball even further.

I have a feeling that Black Goku might not play as big a role as he's coming off. i feel that his presence and involvement will segue into what the REAL threat is. Kinda like when the Androids were introduced and it turned out to be Cell that ended up being the focal point of that arc.