I think this might be a real life reference. Is there a real life counterpart to the Idepon series that is being talked about in shirobako?
It's a reference to an Anime of the early Real Robot genre called Space Runaway Ideon.
Space Runaway Ideon (伝説巨神イデオン Densetsu Kyojin Ideon, lit. Legendary Giant Ideon, also The Ideon) is a 1980 anime television series produced by Sunrise, created and directed by Yoshiyuki Tomino, produced immediately following his most famous work, Mobile Suit Gundam.
[...] At one time it was rated #10 among the greatest anime series as compiled by famous anime magazine Animage (counting popularity, influence, and opinion). On the other hand, the series has never been very popular, even among Japanese mecha fans.
Space Runaway Ideon is a predecessor-of-sorts to Hideaki Anno's Neon Genesis Evangelion and deals with many of the same issues.
[...] Space Runaway Ideon shares a number of similarities to Tomino's Mobile Suit Gundam series. As well as similar initial fates for both series (cancellation followed by films – though Ideon never became a long-running franchise as Gundam did), there are several broad similarities in the make-up of the characters and some reused plot devices. [...]
Wikipedia: Space Runaway Ideon