About Oob becoming Goku's successor, it was established that humans have limited potential with characters like Piccolo, Tien/Tenshinhan etc being left in Goku, Gohan, Goten, Vegeta and Trunks' dust once they attained Super Saiyan.
I guess you could say Goku had more faith in Oob because he was the reincarnation of Kid Boo but it still seems like a stretch.
I don't know whether Toriyama himself would have thought much about what happened after his manga, as of now it seems like right until the end there was no plans.
Toriyama probably didn't want Goku to grow old or pass the baton on to anyone and ended the manga when he did for that reason. Gohan didn't compete in the 28th Tenkaichi Budokai and Goten and Trunks would have preferred to be off dating girls. I suspect the point of that ending was with Goku as a mentor there will be no foes too strong.
The Kanzenban ending narration was changed from
“things will be OK because they have the dragonballs" to
“things will be OK because of all the amazing guys on Earth”, implying there can be more than one warrior Goku trains to help in future fights. Oob was just his newest sparring partner.
Toriyama also retroactively added Vegeta saying he would surpass Goku in the Kanzenban ending, so it seems ultimately if there was anything he wanted to foreshadow its that Goku will continue to grow stronger.
