I really don't think it's possible to make passive stats interesting, they can be made more "desirable" though. For example, if a stat gives you a drastic dps increase or scales really well to the point that it REALLY outshines others then you're going to want as much of the stat as possible. I don't necessarily think that's interesting though. To give you an example, a few years back WoW had a stat called armor penetration, it was the highest dps boost for all classes that dealt physical damage and it's scaling was ridiculous to the point where it became a godly stat. They kept having to nerf it and it's scaling over and over again but it STILL managed to outdo all the other passives, eventually they gave up and ditched the stat entirely in to something new.