I'd go with nobody, but...
Moxley would be my top choice because he has that relationship with NJPW, he will be more than happy to work that kind of schedule, and they have put the US title on him twice before and is arguably the greatest US champion they've had given he's held the title for over 300 days longer than anyone else has. So he will have a lot of credibility there.
Omega would be my second choice if he decides to leave AEW, which is a big if. And even if he does leave AEW he could just be heading back to Japan.
Jay White would be a good option as my third choice, but he's probably in AEW for 3-5 years. So it's going to be a long way off before he would even have the chance to complete the trilogy.
- Chris Jericho - likely never going back to Japan, and even if he did they certainly aren't going to make him their top champ.
- Bryan Danielson - would be a good option but I don't want him working a New Japan style with that schedule.
- Hangman - he just doesn't strike me as a WWE style champ. Probably doesn't really want to go back to Japan either, but that's just me wildly speculating.
- MJF - lol that man is never going to Japan.
- Will Ospreay - it's a non-starter unless he changes his tune on not wanting to live in America.
- Kota Ibushi - his days as a top champ are likely done considering he wants to devote most of his time to coaching. I could see him having an outside shot at the AEW title but there's no way in hell he's ever winning one of the two top WWE titles. And I doubt we ever see him step inside a WWE ring.
- CM Punk - his behavior wouldn't be tolerated over in NJPW.
- AJ Styles - he'll retire as a WWE wrestler unless they decide to cut him or way lowball him on his next contract. And he turns 46 this year, he's going to have to be leaving soon before that AEW title window closes
- Brock Lesnar - only way he heads to AEW is if TK outbids WWE for him. And even if WWE comes in slightly lower, I could see him still opting for WWE because of his history there.
- Okada - I think he has less of a chance of going to WWE than Brock has of going to AEW
As far as others not listed, I don't really think there are legitimate options but two are below. I just don't really see 99% of them even giving serious thought to going to Japan.
- Drew McIntyre - Apparently he and WWE are pretty far off on contract negotiations and is allegedly not happy with his creative. He'd easily be a day-one title contender in AEW and NJPW, so if he leaves then I'd say it's definitely a possibility
- Konosuke Takeshyta - if he ever learns English well enough to comfortably cut a promo in WWE. He's only 27, has a chance to win the AEW and IWGP titles as an AEW grappler. And being 27, and win Japan relatively lacking in higher level young talent, I could easily see them wanting to strap the title on him.