Well, my big picture plan is to have one leader per civ per era. The civs I've consolidated, and Thrace is basically filling the slot for the Slavic peoples in the Classical Era. Therefore, Cersobleptes would conceivably occupy a map with any of the Classical leaders such as Hannibal, Augustus, and Alexander (though Alexander will not appear in my eventual mod, Greece having been merged with Rome and led by Augustus).
What if we simply added some sort of ornamentation to the Hannibal chestplate? Would that be in keeping historically? I'm thinking of a retexture of Genghis' round emblem. It could be colored to match the helmet. That, combined with the lack of pauldrons and sash, could be enough to set the outfit apart. A final touch could be changing the color of the shirt. The leather obviously needs to stay leather-colored.
Alternatively, in place of the central emblem, perhaps the clasps where the shoulder pieces join the front could be replaced with some other type of emblem as well, maybe lifted from the capes on Charlemagne or Justinian and matched to the helmet. (I say "alternatively" because three gold circles on the chest plate might seem a bit much.)
Anyway, it does look good, and I think it fills a void. There really wasn't any leaderhead in existence that I could use as a Classical Slavic leader. I'm considering using Yaroslav in the Ancient slot (even though his clothing doesn't quite fit the period), and I'm thinking your new Dusan for Medieval. Renaissance gets Peter, Industrial gets Catherine, and I'm still debating who to use for Modern. (Stalin will be used for the split-off colony Soviet Union, along with Lenin for the Industrial Era.)
As far as the facial hair goes, I don't know what direction to go there. Again, I've never seen a good picture to go by, but the current clean-shaven with mustache look isn't bad visually. It speaks of both a mainstream European penchant for facial hair and a Hellenistic aversion to it. A goatee could indeed be cool, not appearing on many other leaders. If a full beard is added, some care might need to be taken to ensure that it doesn't simply look like Brennus', Rangar's, or Pericles'. Excellent examples are Ekmek's El Cid and the Doviello leader from the Age of Ice scenario (both use Ragnar's I think). I will likely be using the Doviello leader for Dan of the Ancient Era Germans. Again, even though you're making this for me, you are the artist here. You are the one who knows what can work and what can't, and you've turned out some of the most amazing models I've seen on these forums. I definitely trust your judgment on this.