Well I played the Elder Scrolls Legend and finished the fall of the dark brotherhood quest and I started to wonder are you planning to use the characters from there ?
There is two dark brotherhood characters alive at this moment in Cyrodiil at the same time the dargonbron visits Cyrodill in your project :
The first one is Eliras a Wood Elf Dark brotherhood member It is unknown if he was killed or not. But I think its a good option for possible dark brotherhood story in Cyrodill
The other one is even more Epic its Garnag Orc Dark Brotherhood member he is so epic that even Cicero respects him so it will be nice to find out what happened to him in your project
- He hauled the Night Mother's crypt from Bravil after it was raided, and was one of the last surviving members of the Brotherhood in Cyrodiil.
- In the aftermath of Alisanne's death, Garnag murdered their Sanctuary leader Rasha at the request of Cicero, when Rasha falsely claimed to be the new Listener in 4E 189.
- Garnag later left the sanctuary in 4E 191 to look for food, but failed to return after three months. His fate is unknown.
There is also two options for main Villans in a Dark brotherhood story in Cyrodill the First one is the best in my Opinion its Uther Nere a powerful merchant he burned the sanctuary in Bravil and the last listener. He is a interesting villan. If the player in elder scrolls legends side with him he is alive and it fits perfect with the Skyrim events and Cicero story arc. Not to mention he is really interesting and amazing option for villan the last listener was his own daughter Alisanne Dupre last listener in Cyrodill during the Great war ( Btw Spoiler Uther Nere also killed Alisane Dupre Mother his own wife and Uther framed Alisanne for the murder. really nice guy as you can see ).
So I think not including him will be a huge miss for interesting story that can be made in your project with him.
- Alisanne will die whether or not the Forgotten Hero sides with her, though it is unknown if Uther survives. So you have amazing option for a villan for dark brotherhood story for one of your projects.
He is already established villain in ES lore that won't be used by bethesda so you can write his story.
The other option for main villan is King Barynia of Wayrest but he wont fit Cyrodill . In Elder Scrolls legeds Dark Brotherhood have contract for him but It is unknown whether the King was killed by the Dark Brotherhood, as a contract demanded, or if he was spared considering the events.
Other related characters to Cyrodiil that can be used for Dark Brotherhood quest and even other quests.
Kellen a a Moth Priest that tells the story in ESL :
And Zaria but you know her well that she is in Falkreath
So all thouse characters without the king of Wayrest can be interesting addition to you project and are alive and kicking during the time the Dragonbron visits Cyrodill