Re: Evan Peters Joins X-Men: DOFP as Quicksilver
This is just random speculation, but:
Let's assume that DOFP undoes the "old" X-Men films and establishes a new timeline, with the previous timeline contradictions erased. This would open the way to connect the X-Men and Avengers franchises. There would no longer be a chronological contradiction between the two franchises.
Quicksilver could be a brief cameo at the end of the film that basically establishes that this is a new timeline connected to the MCU.