Depending on who you ask, The Rings of Power was either excellent or it was terrible. On one hand, it had some great things going for it. Elrond and Durin's relationship was a high point, and a lot of the settings and visuals were quite good. Yet, some found the plot rambling, the dialogue cringy and the adherence to lore appalling. With that in mind, it does make sense why the series' viewership declined with every new episode.
The series has some work to do before Season 2, and Amazon has promised just that. In an interview with Deadline, Vernon Sanders, Head of Global TV for Amazon Studios, said this: "We've already announced that [Season 2 will introduce] some characters from the lore. I don't think we've announced all of them, so there are surprises to come." Círdan has been announced, but plenty of other characters could still show up in Season 2.
Celeborn Is Too Important To Leave Out
Celeborn was Galadriel's husband. In The Lord of the Rings, the two were married during the Second Age, and they remained together through the end of the Third Age. However, on The Rings of Power, Galadriel told Theo that Celeborn was dead. So, either Galadriel was misinformed, or he wasn't actually dead. Alternatively, Celeborn could be dead and be resurrected, just like Glorfindel, who was sent back to Middle-earth after his death. Either way, Celeborn is too important to not show up.
Glorfindel Was Resurrected During the Second Age
If Celeborn doesn’t usurp Glorfindel's storyline, then Glorfindel needs to be a part of the story. By mistake, Tolkien made two separate Glorfindels. The first one sacrificed himself during the Fall of Gondolin, and the second one rescued Frodo from the Nazgûl. Because Elves weren't allowed to reuse names, Tolkien explained that Glorfindel had been resurrected halfway through the Second Age and sent back to Middle-earth with the Blue Wizards.
Oropher Was Legolas' Grandfather
Oropher was a Sindarin Elf who lived in Doriath during the First Age. After the War of Wrath, he moved East and settled in Greenwood the Great. There, he was taken as king by the Silvan Elves. For the most part, he and his kingdom were isolated from the events of the world, but when the time came, he answered Gil-galad's summons and joined the Last Alliance of Men and Elves. Oropher was killed during the Battle of Dagorlad.
Amdír Was an Early King of Lothlórien
Like Oropher, Amdír was a Sindarin Elf who lived in Doriath but moved East after the War of Wrath. While Oropher settled in the Greenwood, Amdír went further and crossed the Anduin River. He was taken as king of Lórinand (later known as Lothlórien), where he ruled over Silvan Elves. He also died in the Battle of Dagorlad, so his son Amroth became king. Eventually, Amroth would depart the kingdom, so Galadriel and Celeborn would take over and rule Lothlórien.
Narvi Was Friends With Celebrimbor
One non-Elf who could appear is the Dwarf Narvi. He famously built the Doors of Durin at the West Gate of Moria. Narvi was known for his friendship with the Elf smith Celebrimbor. It's certainly possible that Narvi could show up, but it's also possible that Durin IV has usurped his role. After all, Durin and Elrond were the liaisons between the two races in Season 1, not Celebrimbor and Narvi.
Khamûl Will Become a Nazgûl
Some viewers thought that Theo would eventually become a Nazgûl, but that fell through. So, Season 2 needs to start planting a new, villainous seed. Luckily, Khamûl is the perfect candidate. Originally, he was an Easterling who ruled the land of Rhûn. But after receiving a ring of power, he became one of Sauron's most feared servants -- a Nazgûl. Given that the Stranger and Nori are traveling to Rhûn, it's the perfect opportunity to introduce Khamûl as the future Ringwraith.
The Rings of Power Season 2 does not have a release date. Season 1 is streaming now on Prime Video.