on the map Sinthetic there is the Toastmaster. even though it is just a reskin of a skeleton king it could be a nice addition with the permission of the owner, also the Hellishy Hollow Horseman on the map Bonesaw too for the regular HHH

also the salmann from slime and the mobster skeletons from spineyard are an idea but those aren't really bosses so i'd understand why not