As of now we have: the Power Rangers, TMNT, Planet of the Apes, Star Trek, and The Shadow all interacting with the 'mainstream' DCU... Tbh, I am not at all sure if these are regarded as canon to some degree(as is the case with the He-Man crossovers), or if they're all strictly non-canon. /shrug
...But if ANY of them are canon, I hope to God it's The Shadow. He could add a ton of good stories to DC. 👆