Could it be possible that the time line quests are like chain quests? Each chain involves a saga?

I guess that couldn't explain why Nappa is on Namek.

I'm thinking the Dark Eye saved them (Nappa and Vegeta), and somehow allowed them to defeat Goku. Then we all know where they would go next... Planet Namek to get the dragon balls and wish for immortality.

Here's one idea for what that quest could possibly be about:
The quest would involve something like stealing the dragon balls from whoever has them, and then you would have to fight that character eventually once he tracks you down with their scouter and are super mad for taking their dragon balls.
And you would have to do it in a quest chain that increases in difficulty for example:
Defeat Nappa, Defeat Vegeta, Defeat Dodoria, Defeat Zarbon, Defeat the Ginyu Force and finally Defeat Lord Frieza.
Once you defeat them all and collect all 7 Namekian dragon balls your character would wish to restore the Earth Nappa and Vegeta destroyed and then restore all of lives lost in the battle against them. (don't know if that's possible since Porunga only restores one life at a time.)

well if dark eye helped raditz nappa and vegeta beat goku then they would never had gone to namek in the first place because they could just use earths dragonballs. since dark eye grunts are with vegeta in that one video then they might just be employed by frieza or better yet are aiding the saiyans in over throwing him. imagine having to team up with frieza to stop a bigger evil...that would be a fun twist.

They don't have to defeat Goku for Nappa to still be alive, all they need to do is stop Vegeta from finishing him off.

If you remember correctly... Nappa unintentionally kills Piccolo in the battle before Goku arrives. That causes Kami to die which causes the Dragon balls of Earth to turn into stone.
(When Nappa is firing an huge blast at Gohan but Piccolo runs in the way to save him.)