I'm thinking under the restriction of the map staying the same size (more provinces could be added, but no expansion of the total map borders).

I wouldn't mind seeing another German faction to fill up all that rebel space between the Sweboz, Romani and Getai.

An Indian faction also.
For the rest, I'm pretty content with EB as it is, so I'd probably stick to adding more units for already-existing factions.

To me, there'd be no point adding the Chinese. They didn't come into contact with Europe at all, and it would take ages to reach most other factions. Even if you added another steppe horde (Hsiung-Nu?), they'd still mostly just be fighting each other.

I wouldn't mind either Numidia/Mauretania, or simply more units for Carthage and Rome representing these (more mercenaries, essentially).

Hell, even a second Latin faction.