If by #1 we mean Allen's go to guy I wouldn't be completely surprised if that was in Keon's future. For a number of years Julian Edelman, a slot receiver, was Brady's go to guy and saw a ton of balls. Before Edelman it was Wes Welker, another slot. If by #1 we mean the outside receiver to the weak side of the line opposite an eligible tight end or either receiver at the end of the line in multiple receiver sets (say 2x2 or 3x1) that's a different assertion.
Keon is not a dedicated #1 outside X receiver imo, meaning he is not going to take the significant majority of his snaps outside. Actually not many of today's best receivers do eg Davante Adams. Neither is Keon one of those "slot only" guys obviously. He is going to wind up lining up both outside and inside on or off the line depending upon the purpose underlying the play call. The exact percentage breakdown has yet to be determined and will be influenced by a number of factors some of which are unknown and unknowable at this time. I wouldn't be at all surprised if it wound up a roughly 50/50 proposition.
For those who think the slot belongs to Shakir, his snaps too will be divided in some proportion. He can play some on the boundary.There are two types of slot receiver generally. Some are fast and quick, some are big and strong. We know what group Keon falls in. And there is nothing preventing the Bills from using two slots, like the championship Cardinals often did. Boldin lined up inside and in so doing he benefitted from having more room to operate in the middle of the field. The payoff, in part, was that he managed to be one of the games best YAC receivers, even though he was slow. Coleman reminded me of Boldin a lot when he took off for big yardage after the catch.