Ideally, this timeline happens:

A. Doug Jones wins the Senate seat in Alabama, effectively giving Republicans about three weeks to pass the tax bill (Before Jones takes the oath).

B. Flake threatens to vote no on the tax bill unless the DACA fix is signed by Trump. Collins flips to no because House GOP refuses to pass Obamacare fix.

C. Senate passes Flake's bill (Effectively RAC) OR House passes its own DREAM Act that Flake is happy with.

D. Other chamber passes the DACA fix.

E. Trump is forced to sign it before January because not doing so will stall/effectively kill the tax bill.

Voila. Shutdown avoided. DACA fix granted.

What's the chance of this, though. Highly unlikely.

EDIT: Also, Democrats probably would be thinking they would hold ground on Dec. 22. Given this:

Mark Meadows speculates that they may need to do yet another CR into early January, then *another* CR with the omnibus cap numbers, and *then* an omnibus before Jan. 21 sequestration. - @MEPFuller