I wasn't going to comment since the original request was for anime, but since we've already gotten a more general discussion going on here I'll throw in my two cents...
I think any medium or category can become boring due to over exposure. I sometimes get bored with Japanese manga or anime and pretty much stop reading/watching for a while. I've not come across interesting new WNs lately either.
I've been reading quite a few Korean and Chinese works lately. Some have been truly excellent and/or fun. That said, some tropes have already gotten exceedingly dull. In many Chinese works in particular the antagonists can be really cheap and boring.
Overall, regardless of the medium, or origin or setting... I'm looking for interesting characters and an interesting story. It's nice to see a range of options, as just getting the same sort of thing again and again does get dull. I've no problem with morally ambiguous characters or ones who don't necessarily get much personal character development - relationship development or ability development can be interesting changes as well.
To take an old example - Lina Inverse from Slayers. When someone has a motto like "bad guys have no [human] rights" and treats bandits like walking loot boxes, that can be rather fun. I don't find "edge lords" to be fun but someone who is harder to predict or more willing to go to extremes can be more interesting than someone with very rigid morals. Lina Inverse doesn't change much really and I never felt it was a problem for the story.
Character development can be plenty of fun too, when done well. One of the best series I've picked up in the last year is Kubera, a huge epic fantasy. There's major characters in that story that are billions of years old and for sure they're rather set in their ways but they can still get some development. There's a 1000 year old character who's been an antagonist for most of the series but lately he's been getting a lot of character development and it's been very interesting.
But is character development necessary for a good MC? Nope. A relatively new series I particularly like is "The Return of the Crazy Demon" and the MC has been been just as much of a troll in the latest chapters as he was in the first chapter. Sure, his circumstances has changed but he's such fun to watch that I don't want him to change in terms of personality. If it ain't broke, don't fix it
