I read all the Fletch books, and considered Chevy Chase intermittently good in the first movie. Often he felt the need to mug when he delivered a joke, and at those moments he sucked; but in other parts of the movies, he carried it off.
Jon Hamm seems like a terrible choice, but we’ll see.
Y’know who would have been the absolutely most perfect casting of Fletch? Brad Pitt circa about 15 years ago, although he could still pull off playing the character in the later books. He has that bigdick energy and comedic flair that would work perfectly. Too damned bad the Coen brothers didn’t decide to tackle these around 2000 or so. Sigh. Can I go to that timeline, plz?
Agree. Pitt’s character in One Upon a Time in Hollywood, in particular, is very much an older Fletch.
I think Hamm is great, but he lacks the Fletch’s boyishness and slacker vibe. I don’t see him going undercover as a junkie, beach bum or personal trainer/male prostitute like he did in the books. Fletch is also a very physical role - he does a lot of climbing, running and fighting in the books - and I’m not sure Hamm’s up to it.
You have a point there, I don’t think I’ve ever seen Hamm in a truly physical role. He’s more a lounger than a fighter. 
When they say “in the works” I wonder how far along they are. It seems rumors of a new Fletch movie pop up every few years but nothing has materialized. I remember someone (Kevin Smith, maybe?) talking about how for a long time rebooting Fletch has kind of been the holy grail for a lot of comic actors, but no one has been able to make it happen.
I read all the Fletch and Flynn * books back in my youth. I still have the paperbacks on one of my bookshelves. Maybe I’ll start revisiting them this summer.
I thought the first Chevy Chase Fletch movie was okay, but he definitely goes too far into clownishness sometimes to really feel like Fletch. The second movie just dials up the buffoonery and didn’t feel like Fletch at all.
*Though now I see that there was a later Flynn novel published in 2003 that I haven’t read. I did read the two later Fletch novels with his son and didn’t care for them.
I’ve read nearly all of the Fletch and Flynn books, and own several. (My favorite is Fletch’s Fortune.) I’m not a fan of the Chevy Chase version of the character, but I rarely enjoy filmed versions of books I really like.