Warning! Spoiler for the game Devil Survivor Alert!
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...I'm going to assume whoever read after this saw the warning and accepted it.
This is a story challenge for who finds the premise interesting and is willing to write it. What I am going to write are merely suggestions, I don't pretend for you to follow all of them to the letter. Feel free to add your own variations.
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Basically, the plot DS 1 happens in Area Eleven of the Code Geass universe, before canon events when Lelouch finds C.C., and the area under lockdown includes all of the Tokyo Settlement. The Japanese government was preparing for the Trial, but then war came and Britannia learned of it. The Emperor, seeing a possible threat to his Sword of Akasha Plan, decided to continue the plan made by the Japanese government, but also, seeing the usefulness of controlling Demons, to task a representative of Britannia to win the War for the Throne of Bel. So it's the Britannian Army that it's going to keep people inside the Lockdown, regardless if they're Britannians or Elevens.
And of course, when the Lockdown happens Lelouch and most of the CG cast are trapped inside and receive the Demon Summoning Program.
General tips:
-The Demon Summoning Program and other applications works like in DS1, with Equalizer and all, but it can downloaded into any kind of handheld digital device like in DS2. Who made it this time is up to you.
-At the start Lelouch finds himself in the company of Rivalz and Shirley, to mirror the initial teams of both games.
-The Emperor's representative is Cornelia or a Knight of the Round. Choose who you think fits the best.
-If a human or another Demon defeat a Bel Demon it gains his power and become a candidate for the Throne of Bel.
-Lelouch represents the Chaos Alignment (a mild version at least) and gain the approval and collaboration of Chaos Demons like Loki and Lucifer himself.
-Suzaku represents the Law Alignment (a very radical one) and gain the backing of the Angels
-Todouh and the Four Holy Swords managed to sneak inside the Lockdown, and have the backing of Japanese deities
-Lelouch is the protagonist, so his point of view must be shown.