All of these are just cobbled-together personal thoughts, with most names that weren't given stolen from 10-minute searches on wiki, and suggestions can (and probably should) be changed:
My idea for China's history was that the Greeks arrived just as the Han Dynasty collapsed, founding their own "dynasty" which lasted in some form for a few centuries before collapsing in the next round of invasions, which probably occurred some time around 600-700 CE. Those led to a horrendously fractured China.
I thought about it a bit more, and I would strike down the possibility of any Greek-founded dynasties.
The supply lines between Greco-Bactria and China proper are indefensible, not to mention the Gobi prevents such an advance in the first place.
Bear in mind that the historical period after the Han collapse was a highly militarized one;
Cao Wei could effectively fight both Shu Han & Eastern Wu WHILE keeping the northern nomadic confederations in check.
A couple thousand Greeks crossing the Gobi/across the steppes to fight a battle hardened force like that is not happening.
I doubt the Greeks would conquer the Xiongnu steppes on the way there as well.
The idea of some brief Turkic founded dynasties or states is far more plausible; and perhaps a compromise can be reached if these Turks were Hellenized.
Bear in mind that after less than a century or so, they will likely look very different and already be Sinicized as well.
Also, if the invaders arrive at the collapse of the Han Dynasty and the Han are forced south, that also effectively erases the potential for all the disparate Yue ethnic states in the south.
They weren't ever that organized at that point in time. The Hmong state I feel may be able to stay.
But the Yue ones are sketchy as it was around this point in time where they were Sinicized OTL (under Shu Han & Eastern Wu control).
-Tuunghai (dark), the Asian Hansa, which will have Ryukyu, Formosa, northern Luzon, and a couple of the coastal cities.
This one is very implausible, unless you make them Mohists, and even then,
it's pushing it, as trade-oriented forces in the region don't normally form states.
That is more or less a modern conceit from our era of Imperialism.
The name imo, is also kind of nonsensical.
I've given my thoughts on Serica by extension on what I said about Greeks above.
About religion.
Yes, using Christianity and Islam is easy because we have the IRL frame of reference.
There is a necessity to break out of that comfort zone though. With a PoD that far back, it's pushing it beyond the boundaries of what we can recognize.
I've already outlined that a Cult of Mithra makes an excellent stand-in, especially as it was the historical precursor to Christianity in many, many forms.
We can justify keeping Arabian/Semitic polities in the Middle East as well by replacing Islam with a religion revolving around Ishmael.
For China, Buddhism & Taoism remain the standard. And I even expect the Hellenized Turkic remnants to follow the same,
as opposed to continuing Theoi worship or nomadic religions. We have IRL examples to go by, considering the Indo-Greeks became quite fervent patrons of Buddhism.
Japan and Corea will be very much unrecognizable.
Although a Han exile state in Japan would make sense for the setting;
even more so with Corea in fact. Even after the Han collapsed, there was still maintenance on the military outpost of Lelang Commandery in Corea.
The one in Australia could be a more modern development as far as the timeline goes.
but I dislike the idea.
