Ward isn't a potent enough offenisve fighter to regularly get to Jones. He could give him a little trouble on the inside at times (but he's not a relentless pressure fighter, so he's not going to get into that position regularly enough to do a ton of damage), and his jab would keep Jones at least a little honest from the outside, but it's hard to see Jones Jr. not just potshotting him from the outside and winning a relatively comfortable decision (Ward has the durability, defensiveness ability, and caginess to not get stopped). If Ward was a better puncher his chances might increase as one of Jones Jr.'s few discernible weaknesses is that his durability is somewhat questionable, but Ward is far from a puncher. I might even give Kovalev a better chance of beating Jones Jr. than Ward due to the fact that Kovalev, while not being a pure pressure fighter, is more comfortable on the front foot and has real power in both hands, although, Kovalev would also have a much better chance at getting stopped in spectacular fashion.
One would think that the guys who would be the real threats to Jones would be guys who have legitimate KO power to go with size and serious boxing ability. Guys like Charles, Foster, and Moore who had size, serious boxing ability, and one shot power come to mind. Still, I'd have a hard time actively favouring them over Jones Jr. at his peak.
Two other interesting challenges who don't really fit into the last description would be Spinks and Qawi (Spinks sort of does, but his style is weird enough to have him separate, I think). Spinks is an interesting challenge with his strange style. His size, workrate, and power could have been pretty problematic for Jones. I'd give him a reasonable chance. Qawi's pressure game would have been interesting, as well, as Jones did show at times to be uncomfortable when he got backed up to the ropes and forced to fight on the inside.