I'm inclined to agree with the general sentiment here. A big part of the appeal of the PD hook is trying to work around the limitations of what exists in PD and come up with creative stuff surrounding that, so just opening it up to allowing basically any character from media runs the risk of basically turning this into Pop Culture Clash 2. (Not that PCC2 would be a bad thing on its own, but I feel like that misses the point of this thread's concept.)
Though, for the collectables, I'd say a neat one could be what I'll call Source Material, which essentially works like Masterpieces in concept - but with the material from which characters or stages originated (and with no time limit), so you can learn more about the characters in the game. Some things are more obvious than others - books like Dracula, A Study in Scarlet and The Call of Cthulhu spring to mind as well as animations like Steamboat Willie, but then you could also delve into things like scans of pottery and mosaics for mythological characters, as well as other media forms like comics, webcomics, newspaper clippings, and so on and so forth. If it's a PD game, may as well take advantage of the PD!