Green olives are excellent in egg salad. To my taste, egg salad needs something to keep it from being bland: Dijon mustard, hot sauce, olives, salami are all options that work well.
I am from the US but have no reason to think any of them are common here. Olives and salami are ideas I got from The Charlie Brown Lunchbag Cookbook that I picked up from the bookmobile as a kid. It had a ton of surprisingly adventurous sandwich recipes.
All you people saying “no way” should try it. Unless you are barbarians with no taste for olives.










Egg salad and deviled eggs are very adjacent. Anything that works in one, can work in the other.