I've found that with Egg whites, alot of things easily work, one mistake I used to make was do something like 11eggwhites and 1 full egg trying to flavor it. That ended up working against me. See if all you use is Eggwhites they have verylittle flavor of their own and become virtually just a filler, meaning you can flavor them with practically anything because there is very little taste interaction. Some notable things
Just black pepper, (ok for smaller dishes of whites larger ones gets blah by the end)
Sweet n sour sauce mixed w/ the whites, it takes very little sweet n soure sauce bbq sauce ect.. to flavore them, I used to make the mistake of thinking I would need more sauce to flavor 12 whites, and when I came to the last portion of the whites it was so disgusting because the bottom of the bowl would be so drenched in the stuff, VERY little flavoring is needed, since there is no other flavor to offset the taste of the sauce.
Mustard works well, This one actually goes decent if you decide to try and flavor it with a whole or two, but also works w/ all whites, One pretty interesting thing is that to other people it looks just like normal eggs since it ends up w/ the correct coloring as if you've used whole eggs. I've sometimes added the mustard into the whites before cooking them up, its a little difficult to mix but it works, I'm not really sure that theres anytaste difference however. black pepper also works well added to this.
Protein powder thrown in makes a strange kinda pancake, if you put it into the uncooked whites and then cook, So far I've only tried this w/ banna and rasberry flavored whey proteins.
I've heard peanut butter works too I have yet to try it, too me if I'm going to be adding in the fat from peanut butter, I might as well instead use the yolk fat and have a decent egg, but I guess if you rather have the peanut fats or if it takes less to flavor the mass of it, or if your sick of all eggs in general. (mixing it seems a bit difficult)
I'm thinking of trying to flavor the eggs w/ a packet of sugar free jello, to get strawberry eggwhites, I would put mix it in w/ the uncooked whites then cook, I have yet to try it, but it sounds like it should be ok.