Personally, I think there should be channeler style opponents and non-playable style opponents. There could be 8-10 channeler style opponents but many dozens of non-playable style opponents that could come in multiple forms such as the following:
1. Armed City States: Cities that form around some kind of defensive feature, but with no channeler. They are capable of expanding and, with some luck, create proto nations. They can be negotiated with, conquered, or integrated if relations remain good. Sometimes, a channeler might arise in well-to-do city states and become a channeler style opponent.
2. Dragon Clutches: A group of dragons that live together, typically a group of males or a female and her offspring. They can either be noble benefactors that fight evil empires or wicked manipulators that shake down other opponents for tribute. Sometimes, human worshippers or followers can start settlements at the base of their dens. Either way, they can be negotiated with in a fashion that is limited (while you couldn't make a formal trade treaty, you could forge alliances, or call on them for mercenaries, etc.)
3. Bandit Lords and Robber Barons: Small opponents that spring up around cities that do not guard their commerce well. They can be the subjects of limited diplomacy (bribing to leave you alone or, in a more pricey case, hire them to go fight other opponents on their own innitiative.)
4. Hordes: There can be numerous villages of barbaric humans or other humanoids like orcs that on occassion band together briefly to invade and pillage civilized lands. While they are in scattered village form, you cannot negotiate with them, but can conquer / destroy their villages (be careful, you might incite them into a horde to fight you!) When they turn into hordes, they become temporary opponents that would generally be hostile to everyone but you can use diplomacy to "deflect" them toward other opponents or convince them to return home to their lands. Sometimes, they can become hordes in order to flee from their former land and will damage the economy of any land they pass through as they forage to survive (you could invite a horde to settle in your territory if they are simply looking for a place to live, which would put pressure on your arable land but give you the option of assimilating them later.)
5. Monstrous Tribes: Tribes of large monstrous creatures, like trolls or ogres. They can field raiders against other opponents, be they other monstrous tribes or civilized ones. They even grow in strength and generally when they get to a large size, they splinter, preventing them from ever becoming a formidable empire. Typically what would happen when a region becomes too overpopulated with such tribes is that many tribes would be driven out and in some cases end up in a civilized opponent's land. They are generally a nuissance and must be slain, but if you were to, say, feed them on a regular basis it could open up diplomacy options which would allow you to maintain tenuous relations and, in some cases, you could bring them to war with you as unreliable, but formidable allies, and even hire some of them on as mercenary units.
In the end, channeler style opponents are your long term, permanent rivals, which expand rapidly. Non-playable opponents can be powerful, but typically don't became rivals to world domination because they are unable to organize effectively (though a growing dragon clutch might be an interesting game long ally or arch nemesis.) Minor opponents in this fashion would give the game a lot of spice--- having to only contend with the same old empire building foe time and again can just get tedious...
I just might make a fresh thread out of this post