So who's gonna replace the Yuezhi?

[Avicenna]

Sparta would be nice

Sparta was crushed long before: in 371 BC.

Shortly after, they dismantled the agoge (their soldiering school if you like) and lost their helots (similar to slaves) so their whole system collapsed, the elite ruling class were too few and the workers just left. Sparta now had nothing. Not even their soldiers.

[Teleklos Archelaou]

I guess that's how the spartans in the year our game starts managed to push back Pyrrhos from sacking their town and then followed it up with a campaign that faltered at Corinth when their king was slain. It was far less than it may have been earlier, but Sparta was still there, the soldiers were still there (if often fighting as mercs too, to try and make up for the loss of income that losing the helots brought on), and the agoge was revitalized also. The truth lies somewhere between extremes (e.g., 'had nothing...not even soldiers').