Options to crib from DE and which are already pretty advanced:
Scythians & Xiongnu would add completely new styles of play. In DE, Scyth and Xion cavalry can construct buildings for example. EA's versions don't have to be as radical as DE's, but a focus on innovative playstyles would be important. They are "the nomads" after all.
Suebians/Germans would have cheap, weak buildings like the Celts, but also have Ox Cart dropsites. Where the Celts/Brits/Gauls have plenty of armor, the Germans of Part 1 have almost no armor and would rely on speed and attack. Heavily wood-focused.
Those above already have custom building models. Those below crib models from civs already in the game:
Syracusans are pretty advanced in DE and are what I'm working on currently. Although at first blush they'd seem like they'd be just like any other Greek civ, I think we could do some cool stuff with them: a "Hoplite" civ with "Successor" things such as Quinqueremes, Libraries, Gastraphetes, and come up with some cool special techs for them ("Archimedes' Screw", "Archimedes Claw", "Antikythera Mechanism", "Pythagorean Theorem", etc.). Not to mention nice hero and mercenary options. I wouldn't go too weird with Solar Towers or anything apocryphal.
Thebans are 100% ready to go, but might be a bit bland since they are just a typical "Greek city-state Hoplite" civ. I kind of look at them like the "n00b civ" of DE: uncomplicated, but still strong.
Epirotes are kind of like a variation of the Macedonians, but with a larger roster of mercenaries and access to War Elephants.
The rest of DE's civs are out of time frame for EA's current focus or too far away: Imperial Romans, Gothic Germans, Yamatai Japanese, and Zapotecs.
Options from other mods:
Lusitanians: Getting there, but still need a lot of work to get them up to EA's art standards.
Thracians: Not even close to being close to done. I think people know and like the Thracian aesthetic though. Focused on skirmishing.
Garamantes: Would be nice to add another African civ. Still need work.
Mayas: Out of scope. The mod is advanced and already has a lot of nice things, but needs work for EA-standard.