I would love to see HBG produce a Japanese set sooner rather than later (although I hope to see continued coordination with FMG to avoid too many duplicates). Anyway, the following are some of my thoughts:
CV - Japan has a variety of CV’s from which to choose, and I’d like to see at least two classes represented. Kaga, Soryu, and the Shokaku class rank near the top of my personal list of preferences. I also wouldn’t complain if either FMG or HBG chose the Taiho (I think FMG may have expressed an intention to produce Taiho as the Japanese CV choice, but I’m not sure now).
CVL/CVE - I hope to see at least one CVL or CVE for each nation (although Germany did not receive the Seydlitz/Wesser in any of its current sets), and Japan is no exception. I would personally prefer the Zuiho class CVL, followed by the Taiho class CVE (only one is necessary for this set; the other could follow in a dedicated supplementary set).
BB - The two main classes I would hope to see are the Kongo class and the Yamato class (as a note to earlier posters in this thread, the Kongo class were no longer classified as battlecruisers following their post-WWI reconstruction). I know that HBG expressed a preference for one over the other, and indicated that FMG may produce the other choice. Any further IJN BB choices can be produced through a supplementary naval set should such a set be produced.
CA/CL - If FMG produces a CA, then HBG will probably produce a CL, based on past comments from coachofmany. A Nagara class CL would have a markedly differing visual appearance from any IJN CL, and thus may be a good choice for this reason. Another factor in favour of the Nagara class could be the sheer number of vessels represented by the sculpt; 6 vessels of the Nagara class, and 5 of the very similar Kuma class. If FMG and HBG are both producing CA’s, then my top three preferences are the Mogami, Takao, and Tone classes.
DD - If HBG and FMG each produce one of the Kagero class and Akitsuki class, that would be fine, IMO.
SS - I think that only one sub sculpt is really necessary, hopefully the B1 class.
AK/AP/AO - I would be satisfied with one auxiliary sculpt from FMG, but I hope that every nation receives a tanker sometime in the future.
Armour - The two main choices here are obviously the Type 95 Ha-Go and the Type 97 Chi-Ha, but depending on FMG’s choices, the Type 89 I-Go (“Chi-Ro”) and Type 3 Chi-Nu medium tank would work as well (particularly the Chi-Nu)
SPG and Tank Destroyer - I hope that HBG remains consistent in choosing to produce these unit types; I would personally hope to see the Type 1 Ho-Ni and the Type 4 Ho-Ro.
Mechanized Infantry - Type 1 Ho-Ha Half Track.
Heavy Bomber - I am glad to see that HBG will continue to ensure that each nation receives a four-engined bomber; my Japanese choice would be the G5N “Liz”, an aircraft which I have always liked.
Medium Bomber - If FMG is already producing a two-engined bomber (confirmed to be a G4M “Betty”), then I wouldn’t place any priority on receiving a second sculpt).
Tactical/Fighter-Bomber - Between FMG and HBG, I would like to see both the D3A “Val” and B5N “Kate”.
Fighter - Between FMG and HBG, I would like to see the KI-43 “Oscar,” A6M2 Zero “Zeke”, and N1K1 “George.”