I think this is a going to be terrible idea. Very little chance of it working out for the best.
The fandom only works BECAUSE they have so little actual characterization, fans can project whatever personality they want onto the characters, and the tone of the universe they inhabit.
A cartoon series will have to expand on that, and that it automatically going to turn off countless fans who don't like the chosen characterization.
Also by turning it into a cartoon there is big questions about what they'll be allowed to do. Cartoons are stricter on a lot of moral issues (aka 1930s cartoon themes) that Cuphead as a game can play with. If they make it an adult cartoon series it'll block off half the potential audience, if they make it a kids cartoon series it'll be a critical failure I'm sure.
Not to mention the animation is going to take a massive hit I'll bet.