I agree... I hear Corey's reasoning that it would be "artistically appealing" to have Episode 11 take place entirely in Formenos... there's just not enough that happens there. I mean really, are we going to have Feanor and Nerdanel have the same argument five times to pad out the episode?
Hmmm. I don't think Episode 11 is as empty as you fear, but I do agree it has a limited scope. So, here's all the stuff we can have happen in Episode 11, while keeping to the 'focus on Formenos' idea.
Begin where the last episode left off, with the Banishment being decreed by the Valar. Then, we have Finwë's outrage that he has been 'unkinged' and his decision to go with Fëanor into exile. This leaves Indis and Fingolfin to rule in Tirion in his stead.
The building of Formenos. After all, it didn't exist before this...right?
Nerdanel's reaction - we show the audience that she is far from pleased with her husband's actions, though she still supports him in general...just in a 'what were you thinking, you idiot?' way at this point.
Indis' visit to Formenos. We see Fëanor be a jerk to her, but let her in. She updates Finwë on what has been happening in Tirion in their absence. Finwë emphasizes that he will not leave Formenos until Fëanor is pardoned. Nerdanel's reaction to Fëanor's behavior (again).
A messenger from the Valar comes to Formenos to warn Fëanor about Melkor being on the loose, but said messenger is not let in the door and is unable to deliver the message?
Fëanor's growing paranoia. He mistrusts the Valar, obviously, but he very much distrusts anyone outside the walls of Formenos, and his suspicions fall on those within the walls, as well. Words like 'betrayal' come easily to him.
Fëanor trains his sons in how to use swords and fight...and he's very serious about this. This is not a game. Stockpiling of weapons, and the building of the vault at Formenos to store the Silmarils.
Fingon comes out to Formenos to visit, but knowing he's unlikely to get a warm welcome, throws rocks at Maedhros' window until he notices him, and then the two of them ride off on horses and discuss the state of family politics
Edit: Young Celebrimbor spends time with his grandfather?
Huan languishing in the background.
Melkor's visit. Fëanor talks with him from the doorway, but does not let him in. The rest of the place is on 'high alert' fortress mode, and then Fëanor shuts the door in Melkor's face. Nerdanel observes this conversation, and follows up with Fëanor afterwards. We see that she has been brought around to accept his actions, more or less.