Maybe itīs their conection to Mount Wundagore in general, maybe their powers have a mystical connection with the Mountain. I think Pietro may have powers over time as well as speed given a future version of him from his 90īs series seemed to be capable of time travel and in Son of M he had a kind of time travel hability and maybe itīs just a mental block who keep this power contained.
Curiosly this has been the case with Magneto as well, after he has used his powers for a long time and those begin to affect his sanity they shut down or become way harder to access, Claremont and Bendis said this may be because he puts limits on his powers else he would not be able to keep control of them or as a way to subconsciosly keep himself in check. Maybe Pietro has a similar type of mental block given speed was the main power that keep him and Wanda safe when they were young he has limited himself to this power alone.
Agreed I hope he appears in one of Wanda family dinnersI really want the MCU quicksilver to have a cameo in WandaVision...
Reports said he ins't but i wish it is just Marvel keeping it as a secret like when Natali Portman appeared in endgame