Infinity War marks the 10 year culmination of the MCU and while every major characters is pretty much confirmed for an appearance, I thought it would be a nice touch to throw in some of the smaller characters who have been pretty important in getting the MCU to where it is. I'm only talking about small cameos of some sort. I think it would be a nice way to look back at the history of the universe but maintain the focus on moving forward.
 

Peggy Carter

Haley Atwell's Peggy Carter has a pretty good track record of MCU appearances. Outside of the three Cap films she cameo'd in Ant-Man and starred in the short lived Agent Carter series (which was fantastic and cancelled too soon). Seeing as how we've probably seen the last of Peg, it would be nice for her to get a brief farewell in IW.

Phil Coulson (his first name isn't Agent)

Now this one is a little trickier. I know that by most accounts the Marvel TV and Film divisions to not play ball with one another as much as we'd all like them to. Plus, Coulson is kinda dead in the film universe so him making an appearance would need A LOT of explanation, especially if he turned up alone. But Phil was a character in the films first and I think it would be a cool nod to the history of the MCU to have him make a cameo.

Ho Yinsen

This might be my favourite choice in this list. Held captive in a cave in Afghanistan by The Ten Rings, Ho Yinsen helped Tony Stark build the Mark I Iron Man from nothing but a box of scraps. Without Yinsen, there would be no Iron Man. Without Iron Man, well the MCU would be a very different place. It would be a nice full circle moment if he had a part to play in IW.

Abraham Erskine

A pretty similar pick to the previous, Erskine died at the hands of Hydra shortly after perfecting the super-soldier formula and hasn't been seen since.  It would be a nice touch for Cap to glimpse the man who gave him the chance to win the war.

Stan Lee

Even at the age of 94, I think this one's a given.

Face front, true believers!