Well, I was afraid that my post was running long so I cut it short. But if you like, I can explain why I dont think Hal brings much to the table.
First off, Hal is incredibly predictable. You can basically assume that he's going to do the stupidest thing he possibly can in any given situation, as long as that involves flying in fist first by the seat of his pants without any real plan. That can be fun, but when its the character's go-to move it becomes pretty tiring.
I've also seen people say that everything that makes Hal a great Lantern is what makes him a terrible adult. And I think that's true, and I think it offers a ton of great story material. But no one ever uses that dynamic, I think because it puts Hal in a bad light way too much. He can barely hold down a job and he can't maintain any sort of meaningful relationship. Now, Hal's a maverick archetype, and maverick archetypes struggle with these sorts of things. But where Hal comes up short is his inability to make it work in the end. Most mavericks, like Han Solo or Kirk or Dirty Harry, either learn to make relationships/jobs work for them, or they walk away completely. Hal floats in the middle; never committing fully or walking away. And when he does this over and over, it becomes pretty hard to root for him. This has more to do with the serialized format of comics than Hal himself, but the cause is less important than the effect.
Mostly though, I think Hal offers more as a "villain" than he does a hero. As a hero, he's already pretty much hit the ceiling. He's the greatest Green Lantern out there and there's not many places for him to go. He'll never be promoted because the Guardians cant stand him and he's not reliable or well suited to authority positions. And no one wants to read about Hal Jordan; Desk Jockey anyway. But as a sympathetic villain, you take the limitations off. Hal becomes free to act in ways implied by his personality that he otherwise cant, and you immediately "forgive" him for his flaws because he's not supposed to be a "good" guy. So him breaking Carol's heart over and over? Not a source of resentment or dislike anymore, it becomes "what do you expect, the guy's nuts".
I also think that Hal offers more as a villain because of the ramifications it would have on the shared universe and the GL franchise. He's a founding Leaguer. He knows everyone, their secret identities, where they live, who they're with, their weaknesses, their strengths, fears....all of it, and he's just as powerful (or close to it) as anyone on the League. And no one on Hal's level has ever gone to the other side (other than Hal himself for a couple years where he had three showings). I think the sense of.....violation....would be palpable, and that makes Hal-the-villain more interesting because of the effect he'd have on his former allies. And among the Corps, it's another super-star who has gone "bad". How does that affect the GLC, and the other earth-based Lanterns in particular? How do they deal with that betrayal and the emotional fallout? Just the Guardians' reaction to Hal turning sides, and their attempts to make sure it never happens again, would provide years of stories. The other Lanterns get an elevated status and new foil, and Hal becomes one of, if not the, greatest foe the DCU has ever seen.
Notice I've put words like "bad" "villain" and "evil" in quotes because I dont think Hal really has it in him to be that selfish and cruel. If you look back on the early 90's and his turn as Parallax, you can see that Hal is still trying to do the right thing, still trying to be a hero, but his methods have become way too heavy-handed and he's decided that the ends justify the means. But he's not cruel or selfish (any more than usual anyway). To me, that concept was far more interesting than anything done with him before or since. Hal wasn't just a crazy evil person, he was basically the same guy who had just become fed up and was willing to do whatever it took to change things for the better.
If it were up to me, I'd have Hal forced into joining the Sinestro Corps by Parallax, who wants the only person Sinestro ever feared to drive a wedge through the Corps, (in turn creating more chaos and fear). Hal would be forced into an impossible situation; causing chaos in the Sinestros hampers their activity across the universe, but Parallax wants their fear to feed on. Hal has to stop Sinestro, but in order to get out from under Parallax's thumb he'd have to work with Sinestro, quietly, without the entity possessing him realizing it. It wouldnt make Hal a straight up villain, so he could maintain many of his relationships, but it'd still force him into doing some questionable things in the service of a greater good and cause strife among his friends. It'd give him plenty to wrestle with, both literally and figuratively. Basically, something along the lines, conceptually, of Renegade, but good, and with long-term potential.