I'll try Byzantium. I don't have them in my personal Civ 7 vision (not as a basegame civ at least), but I have thought about what their potential abilities could look like.
Though, I will note; I do have Byzantium written down as a subset of the Roman Civilization, meaning that they share some abilities with Rome.
Leader -
Irene of Athens (just so that we have a different leader to the ones already suggested here)
Leader Ability:
Iconophilia - After completing your first Holy Site, receive a
free Relic. Cities with a displayed Relic generate Culture equal to 50% of their Faith output. Great Works and Artefacts generate +1 Faith and +1 Science. +1 Trade Route Capacity each time you find a relic, or discover a tenet for your founded religion.
Civilization Ability: Macedonian Renaissance. Military units can
capture the great people claimed by other civilizations and use their abilities. Each time Byzantium earns a Great Person, Byzantium receives a Eureka (Scientist, General, Admiral, Engineer, Merchant) or Inspiration (Writer, Artist, Musician, Prophet). Every time
another civilization earns a Great Person, Byzantium receives
a burst of GP points towards the next one (equal to ten times the GP points Byzantium makes for the corresponding GP type)
Unique Unit:
Dromon. The Dromon replaces the Quadrireme and has a stronger base attack. After Byzantium has researched Naval Tradition, it gains a unique
Fire Siphon attack against adjacent enemy naval units, setting them on fire. Burning ships take an additional 15% of the inflicted damage on the next three turns
unless they Quench The Flames (a unique action that
forces a ship to use all of its action points but stops the Burning effect). Great Admirals can protect friendly ships from the burning attack's damage but won't stop the fire itself. Multiple burns do not stack. Burning ships also cannot be repaired as long as they are on fire. Land units cannot be burnt, and neither can
modern warships, such as the Ironclad and the Battleship. Fire Siphon needs to recharge after use, by moving the Dromon onto a friendly Harbor tile.
Unique Building:
Basilica: Temple Replacement, unlocks at Astrology. Half price, grants an additional Great Prophet point and has a generic slot for a Great Work of choice, in addition to the Relic slot.
Sigil: a stylized version of the Byzantine flag, Gold on Purple. (alt colour of Gold on Red)
City list:
Capital: Constantinopolis
Other Cities: Antiocheia, Hadrianopolis, Nicaea, Thessalonica, Athina, Trapezus, Iconium, Nicomedia, Laodiceia, Dorylaeum, Ioannina, Smyrna, Amorium, Sinope, Cherson, Ancyra, Dyrrachium, Larissa, Naissus, Theodosiopolis, Myra, Amisus, Mesembria, Odessos, Chandax, Philipopolis, Heracleia, Mystras, Ani, Edessa, Amaseia, Famagusta, Bari, Nicopolis, Selymbra, Lefkosia, Scodra, Chalcedon, Cephalonia, Phanagoria, Xanthus, Philadelphia, Attaleia, Seleucia, Iustinianopolis, Epirus, Theodosia, Kastoria, Amastris, Syllaeum & Isaura.
I really wish I had the room to give Byzantium an additional unique unit (like the Varangian Guard, the Peltast or even the Tagma again), but I think the abilities I wrote down are already more than strong enough as is.