Costume and casting aside (and I agree about the actor for Spike being waaay too old), you know what I think they'll get most wrong about the live action show?
The action. Or rather the lack of it. Cowboy Bebop may be one of the most lackadaisical shows I've ever seen. Long scenes where people are just sitting around, or riding on a scooter, or walking through crowds. That scene in the the mushroom episode where Ed and Ein are hiding in the tent waiting to see people steal the mushroom. Sit, wait, watch, theft, repeat. Or in the movie where Spike is searching for information and it's just him walking around for like five minutes, no dialogue, set to "Diggin". I don't trust a modern live action version to have the stillness and lethargy which the show has at many points. Also, will it have the same impenetrability? Perhaps it's the translations but I still have no clue why that guy was babbling about beans in the movie. He's got like this entire monologue about beans. Will the live action show have a monologue about beans? I fear it might not.