I'm not sure which film yet but I do believe Pietro maximoff will return because 1. Scarlet witch has magic has her power along with telekinesis and mind control. Vision is also powerful enough to bring the died back. 2. Aaron Taylor-Johnson signed for multi-films. 3. I doubt scarlet witch (wanda) can be alone without her brother because they only knew each other before the avengers or baron Straker. So I strongly believe he will be back just not sure when.