Something I’ve recently discovered-
Instead of the traditional pickled jalapenos, use fresh peppers. They taste so much better- and if you remove the seeds, you get the flavor without the heat overpowering the taste of the burger.
Ham? Fried eggs? That’s a whole other sandwich you’re building inside your burger.
I go for different cheeses, sauteed onions, and thinly sliced pickles myself. I’m liking the avocado slice or guacamole idea, though.
Some people really love Swiss cheese on their burgers. I’m not one of them, but they’re out there.
I’ll eat guacamole or avocado on just about anything. I should probably just pack up and move to California.
Personally, I prefer plain old yellow mustard, Cheddar, and bacon on my burger, and probably have the avocado on the side. Speaking of sides, potato salad and coleslaw are great side dishes for burgers, and don’t have to be freshly cooked.
Chili?
swiss cheese, variety of mustards
Bread and butter pickles.
I have faith that your staff will be pissed that they can’t have a brewski. 
I like fresh avacado slices on my hamburger.
Lots of people like Thousand Island dressing, too.
I would vouch for that, but I’ve never had anything like that under the sobriety constraints of the OP.
OTOH, I can vouch that Mixed Grill is great when you just want to stuff protein and fat into your mouth without it being fried to within an inch of its life when you’re out on the town, so I don’t see why formally putting the egg on the meat wouldn’t be just as tasty.
When I was driving from New Mexico to California to go to college I stopped off at a diner and ordered a chili burger. It was not what I was expecting. Out came a open faced burger covered with some meat and beans. I had only traveled a few hundred miles it was like they spoke another language.
I always have likes sour cream and sauteed mushroom on my burgers…
Also, (and this is along the same lines as the peanut butter named earlier) cashews. I know it sounds bizarre, but it’s just DELICIOUS.
It looks like you’re pretty well set, from great to crazy ideas (try peanut butter on a hot dog…it’s darn tasty) but when I saw the OP I thought of my new favorite place, Bobby Flay’s Burger Palace. He’s got some great ideas regarding topping combos. In fact, one thing that I’ve come to love is chipotle, or some kind of spicy, ketchup.
Yes, yes, yes!!! A fried egg with its oozing yolk is perhaps the best topping possible for a burger. How could I forget it?
Mayo and Dill Relish is all you ever need.
Last week I had the Slamburger at Denny’s.
Burger, cheese, egg, Hashbrowns. Very good.
Just wish my left arm would stop tingling all the time…
IMHO, they’re even better if you grill them.
My favorite topping is mushrooms sauteed with garlic, with cheddar cheese and a nice smoky barbecue sauce.
Definitely thousand island dressing. On burgers, I prefer the kind that doesn’t have to much sugar in it - I like the “savory” thousand island vs. the “sweet” kind. Might want to add a little extra salt (maybe even vinegar) in the dressing as well.
Pickles, spicy mustard, and american cheese.
Spicy mustard? Are you some kind of commie hippie freak or what? (http://mediamatters.org/research/200905070031)
“What kind of man orders a cheeseburger without ketchup but Dijon mustard?” said Laura Ingraham, "“I hear it’s delish with arugula lettuce. Yum-Yum.” said The Gateway Pundit, whoever that may be. Not to mention S Hannity’s valuable contribution to our understanding of what makes an elitist.
Thanks for the replies…keep them coming
Gonna add this to the list.
I have a special mixture of powders that I wont change.
I never use pickles ,only because the standard pickle that is ordered is overpowering,I could get some that are less offensive tho.I will add a mild pickle.
Avocado is hard to work with,but I agree it is better,don’t wanna mess with it tho.
Salsa is always available at Burger Island also(because of the fresh deep fried corn chips I make)I will try the peanut butter myself tomorrow and see if its something I want to offer.I used to offer PB&J for the vegetarians, but if word got out, I had to many people wanting just PB&J…It is called Burger Island…Eat a Damn Burger!!!
Sounds great but I would have trouble grilling an egg.
Sometimes I offer Swiss but it aint that popular,will probably have it tho.I always serve a “loaded” potato salad along with cucumber salad.My assistant claims coleslaw burgers are awesome
Added
Well, this is a dinner for guests, though, isn’t it? A “build-your-own-burger” type deal? Why not give them the option? I serve stuff I would never put on my food (like ketchup on hot dogs, for instance, or ranch for, well, anything) if I know it’s a common condiment for guests. For me (and many others), some dill pickle chips, onions, ketchup, and mustard are the perfect condiments for a cheeseburger.