From my copy-pasta archive:
So, considering Gurren-Lagann has ended I can finally bring real proof of my hypothesis that compares it thematically to Princess Tutu.
Identity plays a major role in the finale of both shows, Ahiru and Simon both were heroes worthy of legend yet by the end their final decision is to retire as humble but honest to themselves creatures: a digger and a duck.
The quote: "There is happiness for those who accept their fate. There is glory for those who defy their fate." is pretty much the core theme of both shows.
Nia is more of a shoujo-tastic heroine and as a whole you could call her story and personality a mixture of Mytho and Edel. She is an innocent princess who discovers herself as a doll to an evil destiny but thanks to love overcomes her own status and attains a happiness in the level of glory despite big sacrifices.
On a related note, what changes the script in the first place are the messenger dolls: Nia by letting Simon insert the Core Drill into Cathedral Terra's key hole and Edel by burning herself and helping Ahiru returning to the surface after episode 13.
Both follow theme of characters playing according to a determined script that dooms them to oblivion yet overcome this due to their intense will power and desire to live.