Osumosan (Yaranaika)
New Monk Subclass: Jujutsuka (柔術家)
The monk selects one of the martial arts to train. Jujutsu (Japanese: 柔術, jūjutsu,) is one of the most common, although in Mythic Japan there are already over 2000 styles of jujutsu only.
Gaijin might practice boxing, bartitsu, or savate (I know, the last two didn't even exist in 1500s but hey, this is fantasy!). Many Kung fu -styles have also been introduced to Mythic Japan.
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Martial Art can have falls, grapples, holds, kicks, locks, strikes, sweeps, throws, and weapon techniques: kusari-gama, naginata, (glaive), quarterstaff (bō), tetsubo, three-section staff, tonfa, walking stick (hanbō), and war fan.
New Monk Subclass: Osumōsan (お相撲さん)
The monk becomes a rikishi (力士), a sumo wrestler, using his great mass and strength to grapple and overcome opponents.
NOTE: in Mythic Japan, women are also allowed to participate in Grand Tournament (one or more per year? TODO).
TODO: need to adjust somewhat, didn't notice that Monk already gets Extra Attack at 5th level.
Level 3: Makushita (幕下)
Chankonabe (ちゃんこ鍋). TODO (maybe stew gives Extra Hit Points or something?)
Choose your current style of sumo, select one from these:
Oshi-zumō (押し相撲). (Pushing Sumo) When you use the Shove option with your Unarmed Strike, you can shove targets up to two sizes larger than you. In addition, your Proficiency Bonus is counted twice to the DC of their saving throw (8 + your Str. modifier + 2 × PB).
Yotsu-zumō (四つ相撲). (Four Hands Sumo) You gain Grappler feat as a bonus feat, but without the ability score increase. If you already have that feat, you can choose any General or Fighting Style feat instead.
Level 6: Jūryō (十両)
Two Styles. At this level, you have learned both styles of sumo; you get the Oshi-zumō or Yotsu-zumō, whichever you did not choose at 1st level.
Tsuppari (つっぱり). You can attack twice instead of once whenever you take the Attack action on your turn. Note: your Bonus Unarmed Strike does not give extra attacks. maybe: you get two Unarmed Strikes with your Bonus Unarmed Strike ability?
Level 11: Maegashira (前頭)
Oshidashi (押し出し). You can grapple opponents of any size, even Gargantuan. You have Advantage to all rolls related to grappling and getting out of Grappled condition. Also, you can move the grappled opponent as by the rules, regardless of the opponent's size or weight. You can't move an immovable opponent (such as a Yokozuna, as described below).
Yorikiri (寄り切り). You can use the Shove option with your Unarmed Strike against opponent of any size, even Gargantuan.
Level 17: Yokozuna
Yokozuna (横綱). No force can move you from your square, even if the ground or floor under you is actually removed. You can still be moved by teleportation or equivalent magic - but not by telekinesis. You can have Prone condition if it is somehow inflicted on you or you go prone voluntarily.
Extra Reach. You add 5 ft. reach to your Unarmed Strikes.
Two Extra Attacks. You can attack three times instead of once (or twice), whenever you take the Attack action on your turn. Note: your Bonus Unarmed Strike does not give extra attacks.