but i guess anyone can make their own character design and name it mickey mouse, sort of like what marvel currently is doing to the norse pantheon? or the games being released that are set within the cthulhu mythos?
No, the text string "Mickey Mouse" is copyright-protected. You could make a picture of a rat, label it "Mickey Mouse," and publish it as parody (which is exempt) but you could not call it Mickey Mouse as your own brand, due to the copyright. Similarly, most versions of Mickey Mouse are still covered, and not usable. Mickey Mouse is still Disney's character, but the now Public Domain visual of the character can be represented in other works, with a number of constraints.
The Norse pantheon cannot be copyrighted, but Thor's MCU costumes can.
Everything Lovecraft wrote is Public Domain since 2008 in the EU. It's more complex in the USA, but everything prior to 1923 is in the Public Domain, and post-1923 works are in a complex state for a number of reasons. Seems like it should have all entered in 2018...?