While Peggy Carter passed away during Captain America: Civil War, the fact that time travel has been rumored to be part of Avengers 4, and is certainly possible due to the existence of the Time Stone, has made some wonder if Agent Carter might be seen again, but, unless Hayley Atwell is playing coy in order to hide the secret, it certainly doesn’t seem to be the case.
She says she hasn’t even seen Avengers: Infinity War and thus wasn’t actually aware of the mechanics that would allow her to return.