For some time now new adventures have been released with no collectibles on the map, think Arctic explosion was the first, I know it's possible to add random collectibles to the loot because it happens during the football event.
Most people have a collection of cauldrons and adamantite they are never going to use so why not add a quantity of traditional collectibles to the adventure loot of any adventure that doesn't currently have them.
You could even expand the idea to co-ops to actually make them appealing again but it would probably have to be more than a few collectibles for those.