Sorry I am so late to this party! I really enjoyed the fruitful discussion in the very first episode 0, and think that a lot of wonderful ideas were suggested.
Like
dietlbomb pointed out above, I think there is some harm in introducing too many new characters within the frame narrative, though, as the Silmarillion itself is already a 'cast of thousands' story. The frame narrator(s) should be chosen for maximum impact, while allowing the flexibility of the occasional 'guest storyteller.' Characters should appear for a reason, not just because we like them. And as suggested, the way to achieve that is not to focus so much on 'Who will be the new Eriol?' but rather to decide 'Where will be the Cottage of Lost Play?'
Not to rehash, but here are my two cents:
With this in mind, I would choose
RIVENDELL as the setting of the frame narrative. Rivendell is already cast as a house of lore, and is in many ways an appropriate entryway from the Third Age of Middle Earth to the First. I think the primary narrator should be
ELROND. Frodo's surprise at the Council, when he blurts out, "You remember?" establishes him as our prime historian. He's related to practically every key player in the story of the Silmarillion, both man and elf. He is a descendant of Thingol, Melian and Luthien.....as well as Turgon and Idril, but also Beren, and Tuor, and Earendil and Elwing....and he was raised by Maglor son of Fëanor, he's the brother of the first king of Numenor, he married Galadriel's daughter, he's the herald of Gil-galad, and, oh yeah, he raised Isildur's youngest son and fostered the exiled kings of Gondor. Seriously....what story does he *not* have a personal tie to? And yet, as someone born at the end of the First Age, he learned all of these tales as history or legend himself.
Of course I have my own bias here. I've thought for some time that the narrative voice of the published Silmarillion reveals some strong biases that are likely to reflect some of Elrond's opinions of the Long Defeat. The narrator considers the Watchful Peace ultimately fruitless, because we know about the Unnumbered Tears. The Oath is destined to end in failure, not only because Mandos said so, but because the narrator has seen this. The view is clearly from the end of the First Age looking back. Such 'commentary' can come from someone with Elrond's perspective, who has only ever known Gondolin as Fallen, but remembers the people and places.
Rivendell, though, allows for other characters. Elrond needs people to tell stories to, after all, and he can take breaks

. And thus we can have Gilraen and young Aragorn, retired Bilbo, a discussion with Glorfindel, Thorin and Co. passing through on their Quest to Erebor (ok, w/o Tra-la-la-lally!), Arwen and her brothers, maybe even Celeborn and Merry long after the others have left, or a White Council meeting, etc.
Brandon's idea of having the Council of Elrond as a backdrop for stories in the Hall of Fire is a good one. Unnamed elves of Rivendell with convenient opinions or backstories can be used if we don't mind veering too far into fanfic

. I just think it important not to stray too far into giving every narrative arc its own framing device, as then half the story becomes the framing device. It would be disastrous to incorporate ALL of these ideas. But as long as Rivendell is the consistent setting, it lends continuity and ties any alternating narrators to one single frame. Rivendell also allows the elves to tell their own stories to other races in a peaceful environment.
(I realize this does not leave much room for Sam and the Red Book. If a way can be found to work that into the frame, to show, towards the end, how Rivendell has transmitted the history of the elves to the hobbits, who then in turn pass it on to their children....well, that can be done, surely.)
I am willing to compromise, though, by having some (alternate) framing devices occur within the First Age. If they are used to introduce characters of the Silmarillion, they can be helpful for telling those stories. Then we aren't 'wasting' our time outside the story, but rather spending time within it. The suggestion to have Finrod Felagund tell one of the early tales of the Valar to the Men he first meets was a good one - we introduce Finrod and the Men, establishing the different worldviews of listener and storyteller, and set up the later Athrabeth

. This could either be a flash forward -telling a Creation story early in series, when Finrod is largely unknown - or a flashback, where some of the lore is introduced one Finrod meets the Men in real time. I think it would be a good opportunity to treat some of the mythological material as myth rather than history. And Finrod, like Faramir, is a philosopher

.
Another idea is to have Turgon and Elenwë telling Idril a story as they cross the Helcaraxë. This will give us the opportunity to learn why Turgon hates the Fëanoreans so much, as we will now have met his wife and feel awful about her death (there's your drama in the frame story!) and introduces Idril as a child, since she will be important later. They could be telling her about the rebellion of the Noldor, or any other tale of Valinor. It might be interesting if the viewers don't know why they're on the ice yet when the arc begins, and only as they story unfolds does the betrayal at Losgar become clear. The arc could end with Fingolfin's host arriving in Middle Earth under the first sunrise, bring the frame and the story to the same point.
Other instances like that, where characters in the Silmarillion tell the tales - yes, it's not the same as hobbits or the remoteness of the Third Age - can give our characters in Rivendell a break. Maybe one arc per season can have an 'in story' narrator - not a throwaway narrator character, but someone who plays an active role (though perhaps in another story). And then, one of the 'in the First Age' frames can be someone (Maglor!) telling a story to young Elrond and Elros, setting up the Rivendell frame.
I was really impressed by the first day's discussion on this topic, and look forward to listening to more of the podcasts.