ALthough Loki is the current king of Asgard, everyone still thinks its Odin. Odin mentions when he banishes Thor that there were two kings before him. In the opening of Thor we see Odin leading the fight against the Frost Giants, in Thor: the Dark World Odin's father Bor fighting against the Dark Elves, so we could see Bor's father fighting against another foe in the opening of Ragnarok, possibly the fire demons of Muspelheim.
In mythology, the first Asgardian and king was Buri, who came from the ice. His son Bor then became king after him. Bor had three sons, Odin, Vili and Ve. Vili and Ve died while adventuring to Muspelheim. The brothers had gone to steal the Eternal Flame, which we see in The Vault. Vili and Ve died so that Odin could manage to escape with the Eternal Flame, keeping it out of the hands of Surtur.
In the novelization of the movie Thor, there are two statues of Odin's brothers, confirming their existence and deaths in the MCU. However, from the death of his two brothers Odin gained his power of the Odinforce. This might be contradicted in the MCU because we see Bor using what appear to be Gungnir and the Odinforce in the fight against the Dark Elves.
However, a combination of these story arcs might be followed. The opening could depict Buri and his armies invading Muspelheim and seizing the Eternal Flame, and maybe Vili and Ve and the Odinforce have a different story.
The Odinforce is apparently something all kings have. Gungnir is passed down to the succeeding kings. Bor had it, then Odin, then Loki. But when Thor was going to be king, it was assumed he was going to keep his hammer and rule with that. And plus his lightning he would also gain the Odinforce.
The line begins at Buri, then goes to Bor Burison then Odin Borson then Thor Odinson. Thor declined the role and Loki usurped, but in the comics and mythology next in line is Baldor Odinson, how is not seen or mentioned. While in comics and mythology Thor is the son of Odin and the Earth and is not next in line, in the MCU he is the prince and heir. However, Sif did once have a thing with someone named "Haldor," who is now dead.
Buri was born during creation, and since the lifespan is 5,000 years for Asgardians creation would be no more than 15,000 years ago. The Big Bang is in fact 15 billion years ago, so it is more probable that the Asgardian civilization started 15,000 years ago, if not the Asgardian race.
This timetable of 15,000 years ago fits better with the Dark Elves. They existed before the Universe and wanted to return it to darkness. It would make more sense if their lifespan was tens of thousands of years rather than billions of years. However, the rest of the space community in GotG and the scientists on Earth would seem to relate better to the Big Bang still happening when it did. But the timetable of when the Universe was created to when the Dark Elves started to do something about it is almost the entirety of time since the beginning of the Universe.