Angle could but it would probably be a limited schedule like the one Brock has. I don't see him working full time again, I don't think his body can handle it. I think Jeff has burned his bridge. He has been given way too many chances in WWE. I say keep him out and let guys who actually deserve to be there take any spot he would fill.
WWE does not want Kurt Angle. He has no chance, he tried to re-open talks with them in the past, but they were not interested because of his continuing problems with substance abuse and possibly some sort of psychological problem. He's not the most stable guy.
Jeff Hardy MAY have a chance, if he can be proven to be completely clean. Hardy marks are some of the most devoted marks, and he was immensely over. during his run. I could see WWE offering him a contract if and only if he were proven to be completely 100% sober and all of his legal troubles were over, and if he actually wanted to come back.
The reduced schedule at TNA might be better for his sobriety and the health of his family. He has a daughter now and gets to spend more time at home with her...so there's also the question of whether or not Jeff would ever want to be in WWE ever again.