I think it's important that they help Allen immediately.
Tyrell Williams would be a huge upgrade, immediately becoming our top WR and our best deep threat. He's a huge target- 6'4" that understands how to get open when plays break down and may always be Allen's strength. He's only 27, so he has plenty left in the tank.
I would be happy to add Jesse James at TE. He's improved as a blocker which is important in this offense, but what I really like is his fire. He's gutsy and gritty, and that's always been a part of the DNA in the best Bills teams.
I would love to add Travis Swanson at center if he doesn't get resigned by Miami. He probably will, but he's young, smart, and a really good run blocking center. I think he would be a great fit and could be in Buffalo for a while if things break down in talks with Miami.