actually...
...the ending is deliberately left ambiguous and it opens to many different possibilities...HOWEVER....
...according to the clues we have left, it's possible to suggest that Goku is to leave and become a sort of a godly figure who shall watch over earth(but not just earth), let me explain...
At the very end before Goku goes off with nimbus, we hear the narrator say...
"And now we end the story of the dragonballs, with the hope that the earth will never again see the darkness which brough it to extinction so many times, but if that day comes, there is one, who will step out of the shadows and fight in the name of all that is good and true." (and btw, I wrote it down while listening to a clip of the ending on Youtube).
Now, what does that tell us? The reason Goku goes because he is mostly the one whose existence has brought trouble to the earth. And to get rid off human's dependence on the dragonballs so they can depend on themselves, the eternal dragon takes away them both.
Now..
...When Goku hops onto Shenron, he asks to pay his last visit to some old friends - and he sleeps on the dragon and the dragonballs disappeared into his body. Now, what does that symbolise? The dragonballs are part of him and what do they suually summon when brought together? Yup, Shenron, in other words, Goku - like the eternal dragon has received immortality (hence his body is them gradually brought back to his original self as he literally possess a similar power to the eternal dragon itself).
And the last bit in the quote above... you notice that the "one" is referring to Goku and he is going to fight for the right whenever the earth needs help right? So yes, in this way, we can regard him like a guardian, but realise that he is NOT watching over the earth like a fortune teller who is glued to her crystal ball.XD
Yes, you can also say that Goku is going to train, but the thing is,
why did he say so? Did he completely mean it (I admit, he did mean it in some way but...) Who can't get enough of training and always wants to surpass Goku? ^__~.
And realise that Goku says, "he will be back soon", and Chichi took it literally but we can see that Goku is "coming back" in the sense that he will not foget about the earth but shall look over it like a God.
I hope that sums the things up a bit, and of course, liek I stated in the beginning, there can be different interpretations but this is the the most likely truth.
