I'm hoping the next set of Jr. title defences are Ricochet > BOSJ winner > El Desperado > Ospreay, so I think the winner is going to be a wildcard - since Hiromu's being pushed to the moon I think they're going to have him 'carry' the build of his Dominion match (and since there's also going to be IC & WH title matches he doesn't need a major opponent) building a new star out of his opponent. I think potential guys are
Dalton Castle: Since Global Wars is between now and BOSJ, the NJPW audience is somewhat familiar with him and he's only just over the weight limit (although I'm not sure how strict NJPW are about Jr. weight) he'd do well in the spot. It depends how protective ROH are of him, since it's unlikely he'd win just for a one-off title shot
David Starr: Very unlikely since he's pretty unknown, but he works for ROH and wXw, both working partners with NJPW. As one of wXw's top talents, if NJPW is properly looking to strengthen their ties with them (a good idea since RINGKAMPF would fit right in in the tag division and select singles matches). He wouldn't be the first pick for a wXw talent, but since he can show up at Global Wars at least he'd have some recognition going into the tournament
Jay White: I don't think his excursion is over yet, but if they wanted they could definitely pull the trigger and have him return to win the BOSJ, but if they're planning bigger things for him then he'll probably stay in ROH, as he'd likely be directionless after the title match which would hurt him
El Desperado: I think it's unlikely he wins the BOSJ, but I really hope he gets a title match at some point as he's great in the singles role. They're putting a lot of stock into Taichi because of his heat, but I don't think they'd go so far to have him go over so many other Juniors - but Desperado could. He'd currently be a step-down from KUSHIDA, but a BOSJ win is the perfect thing to legitimise him as a worthy challenger (so much so that he'd probably be better off winning the BOSJ now that I think about it). I just want to see Desperado elevated, and a face-off between the two craziest Juniors on the roster
Zack Sabre Jr: If they're going to hold off on Ospreay then this makes the most sense. I don't think holding the RPW Heavyweight title excludes him, the bigger problem is NJPW's aversion to having heel VS heel matches (which is also a problem for Desperado)
Will Ospreay: I think he's the most likely but I don't want him to win BOSJ as it's unlikely he'd win the title - Hiromu's been pushed so hard I don't see his title reign ending for a while. KUSHIDA's redemption arc is the obvious long term story for the Jr. title but I'm really hoping they widen the scope of the division past him and Hiromu as they finally have enough guys to make the title picture feel properly dynamic. Ospreay is a lot like Ricochet, in that he was one of the hottest things in wrestling, rightfully won the BOSJ, but then didn't really do anything after - but unlike Ricochet he's a signed member of the roster, so it's even more baffling why he's been stuck in multiman tags for nearly a year. I wouldn't want him to win BOSJ 2 years in a row just to not win the title and do nothing, and since he's one of the top contenders in the Junior division I'd prefer he has a title match later down the line (especially since beating him and KUSHIDA in the BOSJ can work as an effective push for whoever does win)