I don't think concentrical rings would bother the A.I. I suppose Europe, America, Africa and Asia start the game fully settled. Given how the A.I. likes to settle in these Toundra and Desert tiles as soon as all the good spots are taken in a classic Earth-map, I'm sure Surface Civ. would rush to the 1st Ring (Verne's) with their Explorer units. Eventually, they would make it to Pellucidar Ring.
In Pellucidar, UCs shouldn't have any problems. Does they start the game with a fully settled Pellucidar or does they have to conquer it ? An empty starting Pellucidar would perhaps be the best solution: SCs won't be there before a while, UCs should have something to do beside war. Explore and conquer Pellucidar sound just fine.
What bores you in the 3rd Ring ? The idea itself of a Lovecraftian zone, or Brian Lumley mention ?
I haven't read much of Lumley's works. I've read a few short stories (
Cement Surroundings (1969),
The Sister City (1969),
Spaghetti (1985)) and half of his Titus Crow novels (
The Burrowers Beneath (1974),
The Transition of Titus Crow (1975),
The Clock of Dreams (1978)). I pretty much loathed the Titus Crow Cycle. Although I like the idea of a human organization trying to fight the Great Old Ones, I'm kind of disappointed of the G.O.O. Lumley's depiction. They're way too human and too attached to moral (good and evil). We're far away from H.P. Lovecraft weird entities so different from us we couldn't hope to comprehend them at all. And who could forget the Good Golden Brother of Cthulhu ?
Beside all that,
Cement Surroundings was a good story. I liked his creation, the
Chthonians since I met one of them in the
Alone in the Dark game.
In this particular scenario, Chthonians provide a nice thematic counterpoint to SCs and UCs. UCs, even non-human UCs, are now part of a classic Pulp/Fantasy setting. Pellucidar is a place of high adventures, thrills and explorations but there is nothing really alien. Lovecraftian creatures, with their strange appearances, their not-really-the-same matters, their unfathomable objectives provide this alien and dreadful feelings that, maybe, mankind isn't supposed to explore the depth of Earth and some things should stay buried.
Gameplay-wise, an Abyss-type Civilization could (Lovecraftian, Lumley's, or not) provides a good late-game game-changing event. Too often, a Civ game isn't really interesting near the end. Major powers are there and can't really be moved. If all Abyssian units (but one, I'll come back to this one later), can't cross the few tunnels out of the 3rd ring, Abyss Civ is stuck until a SC (or a late UC) drills through the closed doors of their tombs, basically building a road. Late game, Abyss Civ could gain access to a worker-type unit unwheeled and able to build a road to free its Masters. With their powerful units, an Abyss could sweep the map, forcing SC and UC unto uneasy alliance or, at least, think again about their immediate objectives.
I don't want to seem to be lobbying for a particular vision of what the scenario may be.
That's just a few ideas I got and I'm only happy to talk about it. That's really some intriguing scenarios you're putting together.