As of this writing it seems like AJ Styles is actually under contract with any company at this time so that means that Ring of Honor and NJPW actually pay AJ per appearance rather then a flat rate for the contract. Since his departure from TNA wrestling almost 2 years ago he has become one of the top wrestlers in the world during that time. But with his contract status now out in the open this means that AJ is free and clear to work anywhere he wants to work. This means that if WWE wants to send AJ offers then he is free and clear to discuss contracts.
Rumors of AJ Styles with the WWE have been present more over the last two years, but have also been around for almost the last 15 years. Styles would be a great in NXT if the WWE does plan on taking the brand on tour more next year then getting more talent is a must and Styles would be perfect. But at his age with a wife and kids, Styles is working where he make the most money. Currently the position he is in with NJPW and ROH, is awesome as he is getting as a top guy in both companies, but at his age it might be getting to the point where he should think about dialing it back.
If Styles was to receive a contract from WWE the type that Rhyno and James Storm received then I do believe that it would be the best contract for AJ at the place he is currently in his career.