This is the big one. And you're right, he would be way above the NXT level and would go directly to the main roster like Styles. He's as big a star now as AJ Styles was, probably even bigger in Japan, and could easily adapt his style to the WWE in the same way AJ did. As successful as AJ's run has been in the WWE, he's a multi-time world champion and the face of the blue brand, Omega could be even bigger. I would aruge that at this moment in time, Omega is the hottest wrestler in any promotion. His matches with Okada last year raised his stock massively, and his reunion with Kota Ibushi has resulted in Japanese crowds reacting in ways I've literally never seen before. Omega will also be around 5 years younger than Styles when he came in, so there's a lot more longevity. Whether you like it or not, Omega is the epitome of the modern wrestler and if I was WWE I'd be looking to strap the rocket to him on the main roster. Stone Cold Steve Austin has said the same thing as well. He really could do massive business for them if marketed properly. He has all the tools to be a top star in the modern era.
That being said, Omega has a certin feeling of loyalty (and rightly so) to New Japan. NJPW is trying to expand globally, and as a billingual star that's super over with western audiences Kenny Omega is a huge part of that. New Japan is also the company that gave him his big break, and he understandably feels obligated to give back to the company now that he's in a position to do so. Plus, there's the dangling carrot of the IWGP Heavyweight Championship. The main differene between Styles and OMega is that Styles has held the IWGP Title (twice), I think before he's done in Japan Omega wants to hold it. The company probably also knows this. Depending on how big a mark Kenny is, that could be possible incentive to stay.