I'd say: use Greek names (with Latin letters!) for all Hellenistic states, except for those whose Diadochi are still alive (another exception: Macedonia). Thus, Makedonia, Thrake, Syria, Persia, Baktria, Aigyptos, Asia, et al.
Diadochi countries: Lysimakheia, Seleukeia, Antigoneia, et al.
Other countries use native or English names (depending on how these coincide with Greek names): Persia, Egypt, et al.
I admit it will be difficult to find a consistency here, but naming a country Seleucid Empire is just ridiculous.
Diadochi countries: Lysimakheia, Seleukeia, Antigoneia, et al.
Other countries use native or English names (depending on how these coincide with Greek names): Persia, Egypt, et al.
I admit it will be difficult to find a consistency here, but naming a country Seleucid Empire is just ridiculous.