Mayo, certainly ripe for additions. Mustard*, horseradish**, sriracha, chipotle, ketchup ("fry sauce" more or less), etc. Most of these are available as products already. Note that mayo is normally made with a bit of mustard, albeit not much; and creamy horseradish is basically a flavored mayo (check the label, similar ingredients!).
I find I don't even use ketchup much anymore. Mainly with fried potatoes. So I don't have a lot of opinion on it, alas.
Sweet Thai chili sauce is a common sight in my 'fridge, having a nice flavor while being as sweet as ketchup, less sour. Don't know that I mix it with a lot though.
Onions and sour cream yeah, you basically get ranch dressing, an American staple; lots of aromatic/cheesy things can be added to further enhance it.
Tim