Witcher1701 has created a mod called Legend of the Sea Dragon - Ryukyu for Europa Universalis IV. Check file details for more information on what this changes.
-Religion changed to Shinto
(You can now peacefully expand through colonisation without Japan bullying you all the time. Plus if you played Ryukyu before you would know that late-game you get harassed a lot by pretty much every other nation because of your shamanist religion. and who likes religious wars anyways?)
-Slightly increased base tax in Okinawa. (5)
-Slightly increased manpower. (2)
-Better ruler at start. (4/2/2)
-New culture group: Ryukyuan
-Completely compatible with every mod. it uses its own files and doesn't mess with the default ones. Yes, this means you can use it with any
mod that overhauls Asia but probably not if it directly changes Ryukyu in major ways. I am unsure.
-Unique national ideas
-added naval attrition reduction to 'Masters of Navigation' idea.
-reduced 'Masters of Navigation' navy tradition increase from 1.0 to 0.5
-removed +1 colonist from the 'Great Dream' idea. felt a bit overpowered.
-lowered ambition prestige from 2 to 1.