It should not be the case that every faction immediatly after destruction reapeas with an even stronger army on the same spot. It would allow for "new faction" of the same culture to pop up after one faction was destroyed.

For example, the Saka might reappear as the Yuehzi when destroyed under the same triggers that are now used for this invasion, but as a real faction.

The Sauromates might come back as a new wave of Asian Nomads after some time somewhere on the edge of the map, also to prevent the settled factions from controlling the steppe for to long.

KH might spawn next to any Greek town after descrution, that is controlled by the Eleutheroi with a strong enough army to take it and start a new run as this polis.

Celts can come back as any other Celtic tribe, that has not been conquered by another faction, after beeing killed. The same is for the Sweboz on free Germanic lands.

Re-hording with other factions won't make much sense.