Fondue in a little Crockpot/slow cooker? There are lots of kinds of fondues. Cheese, marinara, chili-cheese… I bet people would like that. I'd avoid a hot oil fondue for obvious reasons, but the other ones are probably okay.
If your coworkers usually are good about bringing dippables, maybe bring some dips beyond the usual ranch and salsa. Guacamole's often a winner (I'd stick to a simple recipe, as not everyone is enraptured with bacon and cilantro), hummus or a Tex-Mex bean dip would be good, tapenade or pesto. If you're worried about there being enough stuff for dipping, plain crostini would solve the problem nicely.