Not sure what your plans are for future additions, but some ideas for more more minor classical era civs, and resources I've seen.
CELTIBERIA
I don't know how playable they'd be given they only have around 500 years before getting crushed by Rome (unless maybe you turtle up in northwest Spain), but they'd add something to Iberia. If I remember correctly, you can give leaders an attribute that makes them ameniable to easily join wars, I think it'd make sense for the Celtiberians to represent their role as mercenaries. The Greek states could fight them or pay them to attack Carthage, same for Carthage with Rome.
A few leaderheads for (the Lusitanian) Viriatus exist, but the only working one I could find is
here.
ILLYRIA
Again, their fate is to get crushed by Rome, but they've a long history fighting against the Macedonians and with or against the vairous Greek city states. Records of them fighting the Celts and Etruscans too amongst others, and obviously the Romans. A lot of broken links when looking for resources, but here are two that should work:
Queen Tueta,
Illyria Civ.
KINGDOM OF SABA
Probably the most playable given their timespan, location and history involving trade makes it easy to think of trade related UHVs. I've seen a screenshot of the Queen of Sheba on a thread but no links, but they exist in this RFC Classical++ mod as a full civ with their own leader, though I think the leader is actually from the later Himyarites.
Link.
1SDANi also made Yemen + Oman if they were to be reborn later,
here.
ETRUSCANS
Don't think they really fit on the map, but they exist in a couple of mods including RFC of the Greek World. Would force Rome to fight out of Italy and with the addition of the other civs turn them into serial civ eradicators.
RFC Greek
COLCHIS
Not sure adding Colchis really adds anything to the game given their small size. But if there are still free slots they're probably more interesting and longlasting enough to interact with for other civs in the region compared with just having a generic indie city. A leaderhead exists in
this pack (though it's Aeetes from Greek Mythology)
The same Novator pack with Aeetes also has leaderheads for other civs including Dacia (and RFC classic has it as a civ) and Thrace, but I'm not sure they fit the mod. There's also the Xiongnu in the 1SDANi mod.
This is minor, but there's also a
Troy civ with a UB (and maybe UU) that could be given to the Indy city of Wilusa.