I had a thought regarding the conversation between Future Trunks and Goku, when the former talked to Goku after defeating Frieza.
The whole "Goku wanting someone to succeed him" thought may not have crossed Goku's mind if not for this conversation. At this point, Goku knows that if he died during a fight, the Earth's Dragon Balls will not resurrect him again, but maybe the Namek ones would. But what happens when you die not in a fight, but from a disease, and you know that this is not something the Dragon Balls can fix? In fact, given that all the Z-Fighters, including his son Gohan (Gohan may have died 13 years after the others, but he sadly did die) died because he was not there, that was a shocker already, and Goku must have wondered whether the others stopped training or didn't take it seriously.
This is where Goku might have the start of a realization that his friends might be relying on him a bit too much, and they need to find ways to face the dangers without his presence. In fact, this might be where Goku starts wondering who could succeed him, before thinking about Gohan. Of course, Gohan does not want to succeed his father, because even if he is stronger than Goku, he lacked the fighter's instinct, for which as I mentioned in this post, there is a possible reason for Goku not knowing about.
This is just a thought I had.

















