For Sting, I'd like him to be a 'justified heel'.
He'd have two main points.
First and foremost, he is out to protect TNA. Nearly ten years ago WCW suffered from Hogan and Bischoff refusing to let the younger talent shine. Instead, Hogan would shove himself and all his friends down everyone's throats. Sting will even state that Hogan is doing it now with Abyss instead of sitting back and letting the fans select who they want. Sting wants TNA to live, not die like WCW. In short, Sting blames Hogan for the death of WCW and has waited nearly ten years to confront Hogan about it. And when Sting was finally able to confront Hogan, he did so with a bat.
Second, this is a matter of respect. Six months ago, TNA was very respectful to him and very appreciative for what he has done. Now, Sting has been given no respect. When he had to miss the Jan. 4th show due to a family emergency, Bischoff ripped him up about it and Dixie let him. Sting will say he has been loyal to TNA for over five years, through the good and the bad, and even when people said it wasn't worth it, that he should spend his remaining years in the WWE, Sting stayed. When TNA needed him, he was there.
Now, this one time that Sting couldn't be there, TNA, or rather Dixie Carter, ripped him apart. Sting would say he would have thought after five years, TNA would have given him the benefit of the doubt and to trust him. However, as has been the case in the past regarding trust towards him, he was wrong.
In addition, since Hogan's entry, Sting has received only one phone call and that was to tell him to show up on the 4th. Dixie didn't ask but rather instructed him that he needed to be there. In fact, Sting had to call Dixie to sign a new contract. Sting will say that is major disrespect to a guy who has given so much to their company. Furthermore, that is major disrespect for someone who has accomplished so much.
This would add interest to his character. He's not turning heel for some lame reason but rather an understandable, sympathetic reason. Some fans would agree, some would disagree.