Why not have the zone(s) be 15-50 with scaling Igneous levels.
Why not throw in the Devouring Earth, and possibly create a more advanced culture under the ground with the Igenous. I mean they're slightly intelligent creatures so they should be able to create basic shelters or spires. Probably hvae another Trial down there detailing a story of war and history of the Igneous, Circe of Thorns and the Devouring Earth, a history that developed away from the knowledge we have of their world.
Probably have the entrance in the Hollows in the crater, but it would be guarded by a mystic from the Circle of Thorns (who's that one crazy guy in the Rogue Isles that can see the future? I forgot his name). Probably have him guard the entrance and require the Cavern of Transcendence Badge to access the world beneat Paragon. The world would need to be relatively seperate if it would be 15-50 having individual zone details the CoT/Igneous have sealed off, and by sealed off I mean invading Devouring Earth sectors that the Igneous/Circle are unable to repress with their every growing numbers.
Probably have a firendly CoT faction that were not controlled by the Demons, or maybe even have a truce between you and them because of the looming threat of the Devouring Earth.
The way I see it with the birth of the Hamidon his control did not just expand over paragon, it expanded under it. From this expansion the Igneous and Circle were threatened with loosing not only their homes, but their lives in the process to the Hamidon's minions. In desperation the mystics of the Circle of Thorns magically sealed off the inner sanctums of Oranbega and are completely at a loss on all sides. Those who weren't locked inside their inner sanctuaries slowly aliented themselves from the Circle and allied themselves with the strange, but slightly intelligent Ingenous creatures. Having one goal for survival they have both begun offensive assaults against the Hamidon but are still at a loss, and as such have sealed off certain areas from the main tunnel systems they now have intact.
By proving yourselves in the Cavern of Transcendence you have shown the Circle (and the Igneous) you are a powerful and kind force, but also a force that could potentially save this "Underworld" from a totaly take over by the Hamidon. In desperation the Circle guarding the access tunnel allows you to travel deep under Paragon City to battle with the forces of the Devouring Earth. Unfortunately they aren't the only problem as both Igneous and Circle Mages are still rooted deep into their history. With your recent victories against the Igneous some may feel resent, while the wild Igneous feel hatred.
Yes, there are even wild areas beneath Paragon City with wide open caverns (much like that of the Eden trial), expansive tunnel systems (enlargened to prevent players from feeling cramped, but to also be seen as a height change so that the Igneous themselves can get through).
There would be at least two Task Forces ranging from 15-20, and again from 30-35.
Finally I see a challenging, quite rewarding, and very in-depth Trial that takes you into the depths of Oranbega to release the Circle Mages and use their portals to travel to the Hamidon's influence deep within the recesses underneath Eden. Unfortunately all is not well as something strange has happened to the Circle Mages while being locked up inside the prison they caged themselves in. Discover the source of this unusual anamoly, battle the Hamidon at its root and potentially saving Paragon City (and the Igneous caverns along with the Friendly Circle Mages) from the cataclysmic intentions of the Hamidon itself.
It's all just a thought really, but I think Icould improve on it.
Hmmmm, back to the drawing board. I think the first Task Force could deal with the "Greater Troll Threat" underneath the Hollows that has been kidnapping Circle Mages and destroying Igneous.
The second Task Force would most likely entail the Hydra to a greater extent. Possibly root out the Hydra's angerous rage and devious plot the literally drop Paragon City underground burrying its citizens and hopefully returning home.
I'm better with endings, and I can already see the Hydra broken through a wall in the Sewers hanging down blasting out Mind bullets all over the place as Hydra minions pour from the ceiling while Circle Mages and Igneous repel them. Without the use of the Temporary Guns you get in the sewers I was thinking you'd have to protect Circle Mages while they concentrate on the Hydra beast using Dark Magic to weaken it for the heroes to stop it.
Yup, that's all I've got for now! I agree there should be an "under" zone for us to explore. Who knows, it may even be an expansion one day. (Not my idea but something to do with Oranbega or the Igneous).