Some of the early Phase One characters that come to mind are Dominic Cooper’s Howard Stark, Stanley Tucci’s Abraham Erskine or even Tommy Lee Jones’ Col. Chester Phillips (all appearing via flashbacks, of course).
This could specifically apply to those who were involved in the creation of the Super Soldier Program.
Since at least part of The Falcon and the Winter Soldier seems to center on the legacy of Captain America, it’s possible that we could see returning characters from Steve Rogers’ orbit.
The latter two would be particularly deep cuts for MCU fans.