Aug 12, 4:23pm
As optional Elves, I would add Amdír (a.k.a. Malgalad) and his son Amroth (although Tolkien also toyed with the idea of making Amroth the son of Celeborn and Galadriel).
Also, although Khamûl the Easterling and any of the three Númenóreans would probably make the most interesting characters, and it would probably slow things down too much to go into the backgrounds of all nine Nazgûl, I would still add the other five pre-transformation Nazgûl as potential optional characters.
I would say that all other Men could be optional characters, because we don't know for sure what the focus of the series will be regarding time and place. Otherwise, essential characters could be anyone from Elros to Isildur. However, here are my additional suggestions for optional characters, divided chronologically:
The Mariner's Wife
Tar-Meneldur (Írimon), father of Aldarion
Almarian, mother of Aldarion
Vëantur, grandfather of Aldarion, and the first Númenórean to return to Middle-earth
Beregar, father of Erendis
Núneth, mother of Erendis
Valandil, cousin of Aldarion, first Lord of Andúnië, and progenitor of the Kings of Gondor and Arnor
Silmariën, mother of Valandil and elder sister of Tar-Meneldur
Elatan, father of Valandil
The Mariner's Daughter
Zamîn, caretaker of Tar-Ancalimë
Hallacar, husband of Tar-Ancalimë
Soronto, cousin of Tar-Ancalimë, who would have been King of Númenor if she had not wed Hallacar
Orchaldor, father of Soronto
Ailinel, mother of Soronto and younger sister of Tar-Aldarion
The Dark Years - Sauron's Retreat and the Rise of Númenor
Tar-Ciryatan, son of Tar-Minastir, who began oppressing the peoples of Middle-earth to bring treasures back to Númenor
The Downfall of Númenor
Amandil, last Lord of Andúnië, and father of Elendil (High King of Arnor and Gondor)
The War of the Last Alliance
Meneldil, son of Anárion, and third King of Gondor
Valandil, son of Isildur, and third King of Arnor
Elendur, son of Isildur
Aratan, son of Isildur
Ciryon, son of Isildur
Lastly, depending on where Amazon decides (and is allowed by the Tolkien Estate) to go with the series, I would also add the Elf Glorfindel and the Istari Alatar and Pallando, or Morinehtar and Rómestámo.
Hêlâ Aurwandil, angilô berhtost,
oƀar Middangard mannum gisandid!