I think Balor has a 50-50 chance of being a new breakout star. It may be 75-25 in his favor, because NXT and HHH are fucking hype over him and his aesthetic. It could go either way.
Anderson and Gallows could make a badass tag team. Probably the best tag team in the company aside from New Day. As for success, their success hinges on the aforementioned stable they'll (obviously) be a part of. 70-30 in WWE's favor and that's not because they're not talented. Today's WWE tag team division is booming, so maybe it'll help them stand out and get over as fuck. Who knows.
Aj Styles is interesting. He's a known brand because he's THE TNA home grown star, which raises his WWE ability quite a bit because you know they petty and may want to not give him a chance just because of that alone. On the other hand, AJ is arguably the best wrestler in the world right now - no joke - and he has about 3-5 years left in his tank. If WWE is smart, he could make it. He's such a great wrestler and not that bad of a talker I think he'll make it anyways. 50-50 he gets developed. Depends on how petty they are during that time period.
Shinsuke is the most interesting recruit. He has absolute swagger, and that crosses language barriers. His strong style, if they allow him to bring it to WWE, could develop a STRONG (lol) following because it's rough and fucking RAW. But I think he'll be fine with Kana, Daniel Bryan, and Sasha Bank's movesets. But will be see running knees like this?
Who knows. As for his potential. like the others, he could sink or swim. He has a lot going against him. 1. He's not white, 2. I'm not sure of his language capabilities but I think I read him talking to JR before WK9, 3. His style is raw and vicious and could be heavily tone downed. 60 - 40 in WWE's favor.
Honestly, I think all of them will do alright.
What's more worrying is New Japan Pro Wrestling. The recent acquisition strikes a HUGE blow to the company. Now the only person worth challenging Okada is Tanahashi and that feud is about cooked (I haven't finished WK10 yet don't spoil). AJ Styles had a great feud with Okada, now he's gone. Shinsuke was a great IWGP belt gate keeper, now he's gone. Anderson and Gallows were a huge part of the tag team but diviision but their tag division is fine so I'm not worried. I'm worried because 1. Naito and Shibata have not been built up properly and now they have a gap. 2. They've only got Makabe, Ibushi, and Tana for possibly legitimate contenders to the Rain Maker. NJPW needs to build stars right now with this talent departure. I know they do it, but it's still worrying about the future of Japanese wrestling. This could be a temporary big blow, especially if even more people sign on to WWE.