Just finished listening to the whole podcast episode. My thoughts? I'm excited to start adding these figures to my overall collection with

and

next year, followed by

in early 2026.
As for the TBA figure following Big Bird... I would guess it could be Cookie Monster since that's the one figure from the Toony Classics line still missing from the Ultimate line. Plus, it makes sense since we're getting Grover, Herry, and Frazzle through the year to have everybody needed for :Fuzzy & Blue (And Orange)". On the other hand, I'd really want to ask them what the plans are regarding the NECA letters. Remembering one of the first talking points concerning this line was that the company wanted to make letter figure pack-ins to spell out their own company name. We've gotten the letters C with Ernie and A with Bert, it was mentioned that Mr. Johnson would have an N. That leaves only the E, and another talking point from those early days last year was that it would be the blue female E from the Letter of the Day Pageant sketch... Which led to speculation to her being paired up with a release of Guy Smiley. So I'd want to ask if those are still the plans, then that could be another viable option for this mystery "Volume 5" release.
As far as the rest of the line we know of so far... It was confirmed that

will come with the letter T, as well as Slimey the Worm and other accessories. Unfortunately, the Pachyderm Four represented by the four elephant trunks are potentially not being included here, but maybe and I stress "maybe" in an accessory pack.
Frazzle Monster with an orange U with eyebrows? Oh, that's the U from the sketch accompanying the Sesame Street version of the song "When I'm Calling U".
Sherlock Hemlock with Watson and letter X. Watson might also get cut if the costs for producing Sherlock's coat end up being higher than expected. And a hot pink X? Is that the X from the sketch for the song "How I Miss My X" sung by Patty LaBelle?
Grover and Mr. Johnson still planned as a 2-pack. Grover will have bendie arms, a towel for his waitering role, and a banjo for when he's working at the park; Mr. Johnson will have a tray and bowl of alphabet soup to recreate one of the iconic sketches at Charlie's. No mention of which letters will be packed but we got word earlier this year of a G fro

and an N for Mr. Johnson as stated above.
Herry Monster, still no word on which letter he'll come with. The fact no mention has been made makes me wonder if we'll get a letter (in my mind should be H) or a swerve and have him packed with a number instead.
Looking forward to kicking off this collection and what gets announced at Toy Fair in a couple months.
Edit: couple other points.
Since Sherlock is the first figure with an actual hat, I wonder if it'll be a magnetic attachment or otherwise.
Oscar might not have his elephant pets, but he'll still come with a boatload of accessories so it balances out.