The tone of the game is somewhat atypical of fantasy tropes popular today. The game does not emulate the grounded low-fiction or even the most popular expressions of high fantasy. What I see reflected most in the game is mythological fantasy. Rather than a traditional fantasy party, the characters represent a group of emerging gods. Rather than turning to Tolkien or Forgotten Realms, you should look at mythological sources.
|Details of the Mosaic with the Labors of Hercules -- Carole Raddato (flikr)|
The most obvious source would be the various versions of the labors of Hercules. The character development of the game builds labor-like structures into the heart of the game. You can look at traditional discussions of mythology (Bulfinch, Hamilton, etc.). Alternatively, you can look at different takes on similar material. I think the old TV show "Hercules: The Legendary Journeys" with Kevin Sorbo would be a useful inspiration for both stories and tone (big action / a little cartoonish).
This is also a great opportunity to read into the mythology of cultures with which you are less familiar. If you have not read the Epic of Gilgamesh, this would be a great excuse to do do. Similarly, looking into the mythologies of India would fit the tone well as far as I know (this is homework I need to do).
What is different about a mythological tone (especially as compared to more typical Tolkien-esque fantasy)? This is a little hard to pin down. Certainly Tolkien was basing his work on mythology (especially Nordic and pre-British mythology). But there is a difference. The mythological tales present clearly super-human tasks. Note the emphasis on tasks -- not just victories over creatures, armies, or active opposition. The labors of Hercules involve vanquishing creatures (the Hydra, Nemean Lion, etc.). But they also include tasks that are not combat-oriented. Can you imagine a session focused on cleaning a stable? It was a labor of Hercules. What made it the stuff of myth was that it was a task that required superhuman efforts and dramatic action. You can give your players similar tasks -- mundane tasks with a large scale that requires extraordinary actions to complete.
The tone is also a relatively positive one (usually). Though there are way to seek other tones, the basic setting seems to set up a strongly optimistic campaign. It is not really a question as to whether the characters will reach full godhood. It is just a matter of whether they play through the whole story. The adventures should not really depend on whether they may or may not get killed by a kobold (or its equivalent). Instead, it is about accomplishing the impossible. From Tier 2, players should be attempting -- and succeeding -- at impossible tasks. That is the stuff of legend.
Encourage players to ask themselves whether their strategies are the stuff of legend - or just the stuff of traditional action fantasy. Encourage them to re-route rivers or lift the impossibly large boulder. That is much more heroic than punching a goblin -- even punching them really, really hard.