Help me out: what are the best croque monsieur fillings?

Pictured: The tasting kit at Faste in Paris (which specialises in croque monsieur). Some notable bites here: compté and mushroom, emmental and ham, hot dog and the extremely controversial salmon.

I need some suggestions for how to level up the croque monsieurs in our house because after a trip to Paris I am utterly in love with them (and I’m going make a list so we can have some variety when we inevitably return to this tasty sandwich). What croque monsieur fillings am I missing out on?