After having watched the famous bodybuilding documentary Pumping Iron (1977) featuring Arnold Schwarzenegger and Lou Ferrigno, I think that bodybuilding might not be such a bad premise for an anime or manga series.
The reason I came to this conclusion is that there is an important performative aspect to bodybuilding competitions, and I could see this being depicted in a manner similar to a series such as Ariyoshi Kyouko’s Swan, which is about ballet, or Miuchi Suzue’s acting-themed Glass Mask. The way that both the appearance of the body and the way that posing factors in as a way to highlight one’s strengths and obscure one’s weaknesses can provide ample material to dramatize matches and for an exposition-type character (e.g. Speedwagon in Jojo’s Bizarre Adventure, Ryousuke in Initial D) to go into some extremely granular detail: “His arm is at 47 degrees instead of 44 degrees, causing it to look slightly bigger than it actually is!!!”
I think the real meat of a bodybuilding competition manga would be the freestyle pose-off, where bodybuilders try to make themselves look better and the other guys look worse by changing their poses in response to what the others are doing. There’s the psychology between the competitors and the psychology of how the judges are perceiving the interaction between them, and so it would perhaps be a nice way for a strategist type to psyche his opponents out. When Akagi with Muscles makes his opponent feel smaller, this could even be represented in the art by having a guy suddenly shrink down to half his actual size.
And if you want them to look freakishly muscular, just get Baki the Grappler artist Itagaki Keisuke to work his magic.
Extra points if this were a shoujo manga.