I sometimes mix things in my ground meat and sometimes I just put things on the outside. The only time I have used breadcrumbs is for like turkey or chicken so it's not so hard. Broiled burgers aren't too bad. If I can't grill, I'll broil before I use a pan.
Mushroom burgers: I have used dried mushrooms ground and mixed it into the meat. Sometimes Worcestershire, onions, garlic and parsley in the meat. Top with seared mushrooms, caramelized onions and Swiss. For the buns A1 and mayo.
Cuban Burger: regular burger, ham, Swiss, dill pickle slices and mustard on a Hawaiian bun. Butter on the outside of the bun and press it.
Steak house burger: burger with favorite McCormick grill seasoning. BBQ sauce, broil to thicken. Top with a chunk of sharp cheddar, bacon, 2 onion rings, tomato and lettuce. For the bun, lightly toasted with mayo.
Guadalajara Chicken Burger: ground chicken, chipotle and adobo sauce, bread crumbs salt and pepper. Salsa: cherry tomatoes, green onions, garlic, cilantro, lime juice, salt and pepper. Bun, lightly toasted with avocado and mayo mixed.
Pulled Pork Burger: burger, pulled pork with BBQ sauce, sliced cheddar cheese, coleslaw. Bun, toasted.
Pepper Jack Burger: ground meat, minced chili, garlic and onion, salt and pepper. Roasted poblano, pepper jack cheese, lettuce and tomato. Bun, mayo mixed with a little salsa.
Guinness Caramelized Onion Burger: burger, caramelized onions cooked down with Guinness, English cheddar. Bun lightly toasted, little mayo, Dijon mustard.
That's all I can think of for now that isn't too plain and not too much work.