Jeez dude, I'm gonna be pretty sorely disappointed if Korea doesn't get any updates but this is overkill. 15% tech and idea cost reduction? 20% artillery and 10% morale? These are in line with the highest bonuses in the game.
The reason Manchu gets 10% tech reduction but not Korea is because Manchu is supposed to be one of the strongest nations in the game- it's purposefully given extremely powerful ideas so it gets a shot at unifying into the Qing Dynasty. Now that hasn't worked in game (yet- hoping next patch it will) but by comparison Korea pretty much just needs to sit there and not die.
And for military bonuses... people do often look at the Imjin War through Japan's perspective without much understanding of what happened in Joseon- it wasn't necessarily an inevitability that the peninsula would be overrun so quickly, were it not for some terrible leadership- and likewise, the Joseon resistance after the initial losses aren't credited as much as they ought to be relative to just the Ming running in and saving the day. But even so, it's no lie that Korea's military wasn't remotely respected by its neighbors, and got completely overrun extremely quickly. And then repeatedly, again, during the Manchu invasions decades later. It's somewhat of a misrepresentation to say Joseon's land military was entirely incompetent, but they're
definitely not calling out for some of the strongest land military ideas in the game.
Honestly I think Korea's ideas are pretty much fine. If they start underperforming relative to their neighbors (in 1.19 they're not, but right now they look in a bad spot in 1.20) then I might think differently, but I don't think they really need a buff to them right now.
That said, in terms of a more fitting military bonus than 10% infantry combat (which yeah, doesn't have much place in history), I'd actually go with the new modifier-
land fire damage. Can represent both the hwacha and
Korean musketry, which after their successive defeats at the turn of the 17th century got reformed to actually become pretty formidable.
Something to represent the righteous army and to stop the peninsula from being easy to pacify would be good too, but that should maybe wait until we get a good replacement for hostile core creation.
If we're talking about something unique for Korea to get though (sorry dude but I don't think Korea's crying out for a whole unique government type..), what I'd personally go with is the embracement or rejection of Hangul. It's a situation that's pretty interesting and pretty unique to Korea- essentially, historically, Hangul (the Korean alphabet) didn't become widely accepted after its introduction in the 15th century, because the Korean nobility really didn't like the idea of the general population being able to read- that'd been one of
their privileges. Social mobility was severely lacking in Korea and Hangul threatened to introduce it, so just a few decades after its introduction it essentially got repealed from all government use. It was still learned by nobles, and was used widely by women and for letters to your family etc even while it wasn't used in court, but without state backing it never revolutionized literacy in Joseon like it potentially could have.
So for the mechanic, you could fight to actually introduce it properly. Maybe you could have a series of events, offering you first the choice to invent and embrace Hangul (or just reject it to ignore the potential penalty) which'd give some bonuses of some kind (tech/ideas/development, I dunno, whatever widespread literacy would give). Then over the course of the next 50 years or so, you'd get a series of successively worse and worse events, offering you the choice to repeal Hangul and lose the bonuses, or keep it but piss off your nobility. Loss of army tradition, legitimacy, maybe some rebels, then maybe even top it off with a noble coup at the end. I've never thought about the specific balance but you get the picture, you're fighting to introduce something that'll give you a significant bonus, but have to overcome the inertia of an extremely hierarchical and traditionalist state.
Aside from that, the provinces and terrain in Korea could do with an update-
me and
Bella Gerant made a couple of posts on it just the other day. 1.20 is (potentially, to be fair we haven't seen much of the update yet, maybe more is to come) gonna leave Korea at like a quarter of the strength of Japan, which is a bit silly. Over the course of the game's period Korea would get dramatically weaker and Japan would get stronger, but in 1444 they were to some extent peers. The development levels in game are more in line with the situation in 1910 after Japan had industrialized and Korea'd sat in a hole for several centuries than the historical situation before the Imjin War. I understand there are a variety of reasons to not go with the 1444 levels, but even so Korea could stand to have at least a few new provinces.