Jotaro as a weapon would be a pair of gauntlets that can punch through things as hard as diamonds, and has enhanced speed. These get stronger the better he and his meister resonate.
He would also double as a really great set of binoculars. To use this function his meister would do the whole cupping your hands and holding them to your face thing while wielding Jotaro. If they resonate while doing this, then Jotaro can see things like pulse, body temperature, and anticipate direction and motion.
He would also grant enhanced precision to the miester, to the point where they ask to him to help with really mundane tasks, at which point he would say “Yare yare daze. Make your own damn stuffed chicken breasts.”
He can, of course, take advantage of his enhanced physical capabilities for himself while in human form, but he loses the boost from his meister’s soul.
Design wise, he would have the purple-blue coloration of Star Platinum, along with the shape of the gloves that Star has, but bulkier and more armored. Perhaps he could even have golden cables that spiral around the meister’s arms up to shoulder pad like Star’s, just for the A E S T H E T I C.
In regards to time stop, I kind of don’t want to give it to him unless he achieves Death Scyth rank, but it could work as him having a really rare type of soul. Similar to how Maka has her grigori soul. This could also align with the explanation in cannon for why he can do this, being that he and DIO are the same type.