I have always assumed that the list of factions you can see in the game (at least vanilla MTW/VI that I play) is limited to the playable ones (slightly varying at different periods) plus a few unplayable (Papacy, the Horde, the People of Novgorod in early). All other provinces belong to rebels.
But the other day I recalled my very first MTW/VI game as the Byz (normal, early). And do you think what? I remembered seeing an unlisted faction - the Khazars (in the corresponding province) marked in white on the little inset map in the upper left corner of the screen. They lasted for quite a time and then degenerated into rebels before the arrival of the Horde. Am I the only one with such an experience or has anyone else seen this (or perhaps other) unexpected factions?