As I know I've mentioned more than once, I cannot eat mayonnaise. I've often substituted plain yogurt in dishes like Egg Salad, but it's never been particularly satisfying. Until Greek yogurt became so prevalent. I used Chobani plain yogurt, but it's up to you what brand you like best.
Hard boiled eggs
Plain Greek yogurt
Coarse brown mustard
Salt and pepper
I served it over dark greens with slices of avocado and quartered artichoke hearts.