I personally wouldn’t really agree with the idea of ROK to Japan, I’d much prefer that as a combined Korean tree, or going to US if that isn’t possible due to heavy rejection of this idea from Korean players.
For Japan, the direction I prefer is to go with the Founding ASEAN nations, and build on the early and good relations of ASEAN with Japan to make it a combined ASEAN + Japan tree, while leaving some ASEAN countries more closely aligned with China like Myanmar, Laos and Cambodia for the Chinese tree.
I don’t mind Chinese subtrees. I am just thinking that nations like Pakistan, Bangladesh, Myanmar, Laos, Cambodia and maybe Iran and North Korea would be better options here. In fact I was meaning to work on a multi-national subtree concept for China, though I didn’t get too far with that yet as I had other distractions.
Thailand was actually an F/A-18 customer in the late 1990s, buying 4 single seat F/A-18C and 4 twin seat F/A-18D as their new B.Kh.20 fighter aircraft. This all went well, the first payment done and all in production with Thai serials already assigned to the airframes, when the Asian Financial Crisis hit in 1997, and it was clear Thailand couldn’t complete future payments.
The order was cancelled, with an agreement being made that Thailand would be refunded if another export customer was found. But as Kuwait declined, the aircraft were instead all finished as twin seat F/A-18D models and ordered to USMC service without issuing a refund for Thailand.
So I believe a Malaysian F/A-18D could mesh very well with Thai aircraft, maybe even get a semi-historical Thai camo.
Russia also would have an option for a Russian prototype Su-30MKM that could be added independently of where Malaysia ends up.
I’d also add to this list the Russian Su-30SM that’s being added next update, which is another MKI derivative, and an export variant of it, Myanmar’s Su-30SME could be added for China.