The character breakfasts are going to be your best bet for lots of protein, but if you're wanting quick service...
In MK, the Friar's Nook has a sausage, egg, and cheese bagel that's decent. At HS, Ronto Roasters has a morning wrap that's a sausage link, egg, cheese, and peppercorn sauce. It's quite good, one of my favorite QS breakfasts. In Toy Story Land, Woody's Lunchbox also has some breakfast options that are very good. Epcot has a few options, depending on where you enter. Sunshine Seasons has a meat, egg, and cheese croissant. I haven't tried this one. Les Halles Bakery in France has a quiche that's excellent. In AK, Yak and Yeti local food cafe (Y&Y QS) has breakfast bowls that are pretty good. Satuli Canteen also has breakfast, but I haven't tried these yet.
There's also the option of Starbucks breakfast sandwiches in every park. I like the bacon gouda sandwich personally.