Namely Taipei and South Korea(if they could get it straight), as Ishikawa/Hirano are shaky as a pair and have lost to CSY/CHT and JJH/SYB before. And to a lesser extent, Hong Kong and Singapore, depending how well DHK and FTW play on the day. Germany and North Korea are not the same as the one in WTTC 2016/Rio 2016 but a bad experience can haunt you for years as Ishikawa told in one episode of Table Tennis Japan. Japan has the overall edge here, but anything can happen if they lose the doubles, so best for them to keep an eye out for these teams.
Mizutani said earning the MS bronze had given him a huge confidence boost, which aided him in beating Boll in the MT SF and then Xu Xin in the MT final of Rio 2016. As LGL elaborated in that interview, the XD could become the fuse that triggers the cascade effect. So, if JunMima could reach the XD final and even beat XinWen, and ideally Ito could get the best draw of her career, reach and beat whoever in the WS final, then Japan would stand a much better chance at challenging China in the WT final.
I completely forgot about DPRK. The only substitution from 2016 until now was Ri Myong Sun has been switched with Kim Nam Hae, who is a hell of a player herself. However, I dont think Kim Song I has been as threatening in recent years, but anything can happen.
Germany and Singapore are wildcards too I agree. Germany team is still a bit up in the air. I've heard, but I cant name where, that their team will be chosen strictly by WR (Please correct me if I'm wrong!). Would that mean Mittelham would swap with Shan Xiaona? I don't think that would bode well overall for Germany, but that's my 2 cents.
I find the Singaporean lineup to be a little clearer. FTW, YMY and Lin Ye. Out of the three teams mentioned, I would probably think singapore is the largest threat since FTW and YMY seem to be playing well, albeit YMY hasn't played too often. But yea, it'll be interesting to see.
I'm secretly hoping for Ishikawa to make a breakthrough against China, but im not getting my hopes up haha