If I was to do a 5e version of OE, it would be very true to the 1e version. Every version after that was a pale imitation that failed on many different fronts, from my point of view.
There has literally only been one OA since 1e, and whilst I admit the 3e version was a bit too hung up on doubling as a Rokugan D20 corebook, I still consider it superior to the 1e version, simply because it lacks Gygax's obsession with low fantasy.
First...the whole anime/manga thing? Yeah...just get that RIGHT out of yer noggin' right now. The 1e OE didn't have any of that feel to it. It felt, to me, more like a baseline of "Samurai and Ninja" movies made by japanese/korean/chinese (Yojimbo, 7 Samurai, Heaven & Earth, etc...oh, except for the SHOGUN mini-series that aired on TV....I freaking LOVE that 'movie'!). Now, toss in some "Kung Fu Theater" for some amusingly over the top stuff and PRESTO! First Edition AD&D Oriental Adventures.
Basically, do that, but with the 5e system.
Of course it would lack any "animesque" feel; anime and manga didn't come to America until the late 80s, and Gygax was a big low fantasy fan, so of course he stuck with the kung fu, samurai and wuxia films that began trickling into America in the 70s. That's what Gygax had access to, so that's what he tried to emulate.
Now, I'm not saying that it's
wrong for OA to grant access to that kind of feel. BUT! Different fans have different tastes in fantasy levels, and Japanese/Chinese fantasy is
not monolithic. A good OA sourcebook should provide all the tools needed to support a campaign themed on Seven Samurai (Low/Historical Fantasy), Journey to the West (Swords & Sorcery) or Inuyasha (High Fantasy) - an OA that only supports Low Fantasy is inherently cutting down its audience appeal.
For Martial Arts...definitely would have to have it's own "sub-system". IMHO, just reducing it all to the OPTIONAL (!) Feat system would be whole inadequate. It wouldn't "feel" like I was performing any sort of "martial art awesomeness", and would instead feel like "Oh, yeah. I took that feat so I do d10". Bleeachy! The only thing I'd tweak would be the "effectiveness" of the MA damages vs. weapons/spells. IMHO, if you aren't a Monk, Shoei, Kensai, or Wu Jen...then you shouldn't be doing the same 'deadly 1:1 damage' as some Bushi with a naginanta. Everyone who learns MA (which, lets face it, is about 98% of any OA NPC) would have learned/trained/developed their MA style to beat others. And when I say "others" I mean humans (and 'demihumans'...it is fantasy, after all) style of MA. Some tea house Kung Fu master should wipe the floor against any boisterous drunkard who starts something in his tea house...but when an Oni, Pan Lung, or Bajang pops up? Well, any 'attack' should be much less effective!
My only problem with this is that... what exactly is a Martial Arts subsystem supposed to do? Martial Arts, in the pop culture view at least, revolve around unarmed fighting strikes, grabs, locks, and throws. But... 5e already HAS mechanics for Unarmed Strikes, Grappling, and Shoving (which throws would be a reskinned of), AND it tries to avoid an overabundance of fiddly, super-specific rules. So, how would a Martial Arts subsystem actually offer meaningful differences to these rules, that couldn't be achieved by the use of feats & class features that make unarmed strikes, grapples and shoves more versatile and effective?
Also, don't forget that NPCs don't work on the same rules as PCs; NPCs with special attacks that do heightened damage despite not usisng weapons and/or lead into grapples/shoves easily convey the "martial artist" theme in 5e.
I think a couple tweaks to the three OA non-human races (Korobokuru, Hengeyokai, and Spirit Folk) might be in order....stronger/more drawbacks, but stronger/more powerful 'otherworldly stuff'.
Here's my problems with these three "classic" races.
Korobokuru are redundant. If you actually look up the lore of the koro-pok-guru (the most commonly accepted modern translation), they're described as small, reclusive humanoids who live in the wilderness (in pits roofed with butterbur leaves, specifically), pursuing a quiet existence as hunter-gatherers and fisherfolk with no particular interest in the outside world and generally being inoffensive little people. We have this racial archetype in D&D already. It's called the halfling. Maybe there are stories of their natures, abilities and interactions I'm just not finding, but nothing about what I've found suggests them as anything that can't be covered by giving the appropriate Ainu cultural trappings to halflings.
Spirit Folk, likewise, feel redundant. I don't know what actual myths they're drawing from, I'm presuming they're just generic references to the vast celestial bureacracies of China and Japan, but the worst part of it is that they feel all too similar to elves. Seriously, look at the facts:
* Both races are the weakest and most "human" of a vast array of mystical beings that live alongside yet out of phase with the mortal world (Fey vs Kami).
* Both races look like extraordinarily beautiful and exotic humans with, at most, minor traits that hint at their unearthly true nature.
* Both races live for prolonged periods(? I honestly haven't been able to track down the spiritfolk lifespan in either OA book).
* Both races possess some inherent minor magical traits.
Maybe if the powers the Spirit Folk had were stronger, or they had better fluff than being an Orient equivalent to planetouched, I might have less of a problem with them. As-is, they feel too much like something you could replicate by taking an elf and giving it a racial trait or feat to replicate. River & Sea Spirit Folk in particular feel all too easy to replace with Aquatic Elves. Heck, at least the Dargonesti & Dimernesti have their shapechanging powers to make them unique; Water Folk don't even have that.
Hengeyokai... mostly, my problem with how this race is presented is that it's incredibly weak. It's literally "one trick pony: the race", with absolutely nothing going for it besides the "turn into an animal" power. If the different sub-breeds were better defined and the race as a whole had something more to it than just "you can turn into one specific animal, like a weaker version of a guy who took a level in Druid", I would complain a whole lot less.
Honestly, I really think the absolute best way to handle Hengeyokai is to make Hengeyokai a Racial Trait rather than a Race proper and instead focus on building distinct races who share that trait from the myths. Kitsunes, Tanuki, Bakeneko at the very least have the gumption to stand on their own and be proud races. If we can make dwarves, gnomes and halflings be separate in vanilla D&D, why can't we extend that same courtesy to races who actually do have distinctive characterization, goals and power-sets in the myths? Freaking Pathfinder has done a better job with kitsunes than D&D ever has!
Likewise, I believe that a proper 5e OA should try and broaden the racial options. The myths of Asia's various sub-continents are filled with all manner of humanoid beings and non-evil spirits; plenty of PC fodder.
Classes? About perfect, really. All of them felt like they had their own niche in the world. With the choices for 'customizing' many of them built in (e.g., Kensai and his chosen weapon, Wu Jen and their '5-Elements' type spells, etc), I think each could have one or two Arch-Types would be more than enough to cover anything a campaign may need.
I... really don't understand what you're arguing here, so I can't comment on it either way.
Now, when they get to the 'Campaign Setting" aspect. THIS is where I'd like to see a bit more effort. Chinese, Japanese and Korean are obviously the focus...but India, Mongolia, Vietnamese, Philippines, etc should have at least a couple pages about how to incorporate some of those cultures/histories/fables. Better still, put out a separate "campaign add-on" book/box that could add more in-depth setting stuff for that.
Definitely agree with this.