This is a fun topic. I've recently started going to the gym again IRL but I'm actually enjoying it this time lol. I agree that a rack of free weights or kettle bells would be cool! Also, my gym has a tall rack of medicine balls ranging in weight and colors. A large yoga ball (or whatever they're called) would be cool too. All this could honestly just be decor, because I don't really think working out needs to be updated or more versatile. Just aesthetics. It'd be cool to be able to make more than just the high protein plate and the protein shake to pump up your Sim. Perhaps shakes that have specific enhancements, like lose weight faster, build muscle faster, no fatigue or muscle soreness after workout out, etc.
And of course just lots of gym related decoration like towels, water fountains, scales, hanging punching bags, those busts you use for boxing practice (I think?), shakers, supplements, etc.
I also have been wanting swimwear that is more along the lines of professional swimwear, not just cute stuff lol.