Wait why did this get unstickied?
Anyway, so this won't be a content-less post, here are a few characters who will likely be playable in the upcoming Charlemagne DLC (assuming you own SoI, RoI, and TOG, of course)
Catholic
Charlemagne: Duh
Carloman: The other King of the Franks. He's something of a cipher since most contemporary historians focused on his brother.
Offa, Petty-King of Mercia: I could talk about Offa's extending of Mercian power over Southern England, and how he was arguably the last non-Wessexian Anglo-Saxon king to wield major power over the island, but most people who would play him know that. The real challenge will be to ensure his hegemony outlives him. He also created the Diocese of Lichfield to break Canterbury's power (this was before Canterbury was an Archdiocese, mind).
Desiderius, King of Lombardy: Last King of the Lombards, Desiderius was defeated by Charlemagne (his former son-in-law) and driven into exile. If later start dates are included (and a playable Ragnar Lodbrok has been hinted at, so that seems possible), you could also try to re-establish his son Adelchis on the throne. Desiderius also set up an antipope.
Orthodox
Constantine V of Byzantium, the Dung-Named: First of all, he's called the "Dung-Named", so he has that going for him. Also he was a noted Iconoclast and a military commander with enough skill that he destabilized Bulgaria almost completely.
Tengri
Telerig, Khan of Bulgaria: The reason Constantine's victories over the Bulgars only "almost" completely destabilized the country. Telerig son of Tervel has only been on the throne a year at game start, but he managed to turn around the Emperor's machinations and tricked him into betraying his own agents in Bulgaria. Historically, he was overthrown by Kardam, fled to Constantinople, and converted to Christianity, after which he died. Obviously, you should try to avoid all that.
Norse
Widukind, Petty-King (Possibly Tribal Chief) of Saxony: The last and greatest of the Continental Saxon chiefs, Widukind successfully resisted Charlemagne for years before Saxony finally succumbed to the Franks. You should do one better, and drive the Franks back into Frankland so that a proper, Germanic, empire can be formed.
Sunni
Abd-ar Rahman, Emir of Ummayad Cordoba: At this point Abd-ar Rahman is trying to re-establish his family's power base after the Abbasids threw them out. In time, his descendants would form a rival caliphate that became renowned as a center of learning and culture. But that's easy under CKII mechanics. The real trick is to turn the tables on the Abbasids and reclaim your rightful position!
Indian
Gopala, Emperor of Pala: Founder of the Pala Empire. Not much is known about him, other than that he came from Gaur and - apparently - killed a Naga Queen. It's actually possible he's dead by the time the game starts, depending on which historian Paradox uses, in which case you can instead select his better known son Dharmapala. Both emperors are believed to have been Buddhist
Nandivarman II, King of Pallava: The legends say he was brought from Cambodia after the prior Simhavishnu dynasty died out, as he was a member of the related Bhimavarman dynasty. Regardless of his origins, the young Nandivarman was an intellectual and patron of the arts, who was still militarily strong enough to subjugate other parts of southern India. He was Hindu.