Well, their biggest success is pretty much done to death now. NGE has had a series, multiple OVAs, multiple movies, multiple revamp movies... there's really not much else to get out of it unless you want to tell the story of how they got into that mess to begin with, which might be the most interesting part of all of this to its core fanbase given their age. Maybe tell the story of the project up until Shinji's mom dies or something.
There's all kinds of fun things that could be done with the franchise. An isekai where an Otaku gets reborn as Shinji and tries to do it right would be great.
I mean the plan with Eva according to Anno is to do like what Gundam does.
i.e. you have you main timeline (Universal Century) with focuses on different groups on opposing sides of the conflict during different periods.
But then you have your many other offshoot series (00, IBO, GWitch, SEED, Turn A, etc) that are free of the main established canon while keeping the same core elements to the series.
