They will definitely give the 360 a section on the XB1 marketplace. There was a FAQ that was quickly took down that said they will but not at launch (this same FAQ said release was October, so who knows if this is still the case).
Black Friday sales are going to be glorious with BC. Especially since it's the 360's 10th year anniversary just a few days before it, gonna be some massive sales
. All this on top of Halo 5, Battlefront, Tomb Raider, Fallout 4, Black Ops 3, Need for Speed....... wallet am cry.