There wasn't any noticeable theming for the music when 1.0 originally came out, but both 2.0 and 3.0 showed the pattern of Anno and Sagisu going back to their previous collaborations and reusing tracks. Looking back on 1.0, we could now say that maybe it just had some "stealth theming" by reusing NGE tracks. So...
1.0 - Neon Genisis Evangelion
2.0 - Kare Kano
3.0 - Nadia
More interesting to me though, is that the works that the tracks came from seem to mirror some of the themes of the three films themselves too.
1.0 - Still basically paralleling the tv series, thus inconspicuous
2.0 - Introduces new romcom elements similar to Kare Kano
3.0 - Introduces Wille, which parallels the intentions and aesthetics of the crew of the Nautilus in Nadia
So, I'm thinking that the final film will be...
3.0+1.0 - End of Evangelion (+Death and Rebirth maybe)
End of Eva Speculation:
3.0 ends in a similar spot that would lead to 3.0+1.0 being "the End of Eva of the Rebuilds" anyway, with all the angels defeated and Shinji in a clinical depression type state like at the end of episode 24.
Also, Sagisu himself has been posting things about the music from End of Eva while making the sountrack for 3.0+1.0:
Tweeting a video of the guy who adapted Komm, susser Tod to English singing the song himself.
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— twitter-user (@username) date
Having LOREN from the original End of Eva sountrack at the recording studio.
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— twitter-user (@username) date
↑Both of these tweets were also tagged #シンエヴァ/#ShinEva.
Bonus because it is from Death specifically:
Tweeting a video of the orchestra playing a Kanon D-Dur remix. (It is a classic song, so it isn't impossible that they all just came to the studio knowing how to play it and did it for funsies at Sagisu's request, but I would assume that proper etiquette would be for the composer not to treat his orchestra like his own personal coverband and only ask them to play songs on the set, the ones they plan to record and use.)
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— twitter-user (@username) date
Utada Speculation:
I for sure think Utada will have a song in it, though it is a little wierd that we've seemingly never gotten even a hint from her about it. They confirmed over two years in advance her return for Kingdom Hearts III, and as time went on she even made joke tweets begging Square Enix to finish the game so she could finally release the full tracks.
I'm leaning towards it being a brand new track that will come out as a single too, but...
The potential from some "trolling" in the plot might mean we end the film at the "1.0" part of the title, so they might bring back Beautiful World one more time to send the Rebuilds off. If 3.0 and 3.0+1.0 are truly supposed to be their own "saga" in a way too, it would then retroactively make more sense as to why Sakura Nagashi was used instead for 3.0, since it would effectively be the Thanatos mid-story ending credits from End of Eva.
Galaxy Brain Speculation:
Eva isn't a stranger to licensing songs from outside of even Khara and Gainax.
-the songs on Shinji's Walkman
-the song from the film "The Man Who Stole the Sun" in 2.0
and most importantly...
-Fly Me to the Moon
Hear me out.
Fly Me to the Moon was released in 1954
Neon Genesis Evangelion was released in 1995
A 41 year difference
HEAR ME OUT.
Twilight by Electric Light Orchestra was released in 1981
Evangelion 3.0+1.0 will (hopefully) release in 2020
A 39 year difference
I think they could totally pull it off.
Sending off Eva with the song Anno and his buddies sent off their doujin anime careers to before starting Gainax. Anno himself even sent off his Animator Expo project with Cassette Girl, which was just a big Daicon love letter.
And personally, I think it is would be the perfect song for an inverse Komm, susser Tod sequence, the world turning blue as it bangs in the background.
As for my personal hopes. Yes. I would love some End of
Eva tracks. Specifically, yes, Komm, susser Tod. I'm not sure if it could work with the new story and where the tone is currently at, but if they made it work somehow, that would be cool.