So this is kind of a tall order, since it would fundamentally change the balance and even the overall focus of the game to be far more "Eurasian" than it currently stands (and as it stands it's pretty damn good at including Asian factions, at least from the western half of the continent). However, arguably two of the biggest empires of the 3rd century BC aren't included in EB II due to the focus being more Western, Hellenic-centric and more in line with what everyone learns in regards to Classical Antiquity. The Mauryan Empire of India was easily the rival of the Seleucid Empire in terms of size and population and had steady interactions with the Greco-Bactrian Kingdom. The latter, via the travels of the Han Chinese diplomat Zhang Qian (200 - 113 BC) and subsequent relations, also had contacts with the Han Empire of China. The Han Dynasty was established by 202 BC, yet when the EB II timeframe starts in the 270s BC China is still split among Warring States, the most dominant being the State of Qin, which slowly conquered all of them to form the short-lived Qin Dynasty that ruled all of China right before the Han. The Han Empire eventually expanded into the Tarim Basin of Central Asia, fighting battles against their nomadic archnemesis the Xiongnu as far afield as modern-day Kazakhstan during the 1st century BC. In the late 1st century AD they even had a small war against the Kushan Empire of northern India and Pakistan over a failed marriage alliance and the struggle to maintain influence over the oasis states of eastern Central Asia.
Although it would detract a lot from Europe, North Africa, and West Asia, which are the regions EB II currently focuses on exclusively, the addition of India and China in a submod would be very interesting, because it would lend a fuller view of what was really going on across the world in the 3rd century BC. Yes, there would have to be less regions in the West, but this could always be balanced in a smart way that still makes the campaign of any faction difficult in terms of expanding one's borders. Luckily, Medieval II mods like De Bello Mundi (DBM) already have a massive amount of Mauryan Indian units ready for use, even Han Chinese mercenary ones available to the easternmost factions of the mod. So there's some collaboration with their team and sharing that could be done in order to flesh out new factions. There are also several Chinese history mods floating about on TWC, including the Romance of the Three Kingdoms mod where some units could potentially be borrowed, although they belong more to the 3rd century AD (i.e. the end of the Han Dynasty) than they do to the 3rd century BC, the Warring States period.
Kudos to anyone who earnestly takes up this idea and makes it happen! If expanding all the way to China is a bit much, though, simply the inclusion of Mauryan India + the availability of Han Chinese mercenaries in the easternmost portions of the map (just like DBM) would be a huge boon to the game. Importing these units and other stuff wholesale from DBM could also be an idea worth looking into, since the developers of those mods allow the use of their content so long as you simply credit them.