Will Smith??? No, no, no, no, NO!!!
First off, Cheadle isn’t spindly, not in the least. In fact, that’s precisely why he’s perfect for the role - he’s muscular, yet in a lean, sinewy, feline, pantherlike way. The Black Panther isn’t merely a muscle man, he’s agile and dextrous.
Will Smith is Stallone-style buff. That’s wrong for the Panther. Additionally, while Smith is a highly charismatic movie star, he’s not a particularly good actor. That is, he really can only play one part - that of a hip, urbane, irreverent, street-smart American tough guy. He’s good at playing that character type, but that’s the extent of his range. Cheadle would be much more capable of depicting the nobility and reserved demeanor of a central African prince.
Will Smith as the Panther would be “Halle-Berry-as-Storm” bad.