warning: many spoilers follow
You can be Avatar more than once in a rowSatis had faith in Molly, but deep down he was uncertain about whether any of the racers had it in them to govern the entire Galaxy. Thus, he entered again as Super-Racer, aiming to have the seat of power for himself once again, if only to keep Canaletto out of it. When Molly defeated Super-Racer, he dropped out and acted as an overseer for the Earth Team all throughout the Oban cycle.
O was the vice-AvatarIn a flashback, we see Satis trying to use his magic to counter Canaletto's and crying to O for help. Thus, O had originally planned to race for the seat of power himself once his superior's turn ended. He even scored high at first. But as Satis became more and more determined to have Molly take his place, he instructed O to start throwing the races to allow her to step ahead. That's one of the reasons he helped Molly, Jordan, and Aikka out on the second-to-last episode. A racer intending to win would have headed there in a hurry to nab the Ultimate Prize for himself.
Sul isn't deadTechnically he was sucked into a portal, possibly becuase Canaletto knew that even he didn't have the power to outright kill him. So he compromised by simply removing Sul, possibly trapping him in a dimensional prison of sorts.
Sul wuld have been a great avatarHe already seemed competent with his own power, and needed a bigger challenge, killing an omnicidal maniac as powerful as Canaletto without breaking reality is a pretty big challenge. Also managing a planet that makes planets is really hard, you have to calculate hundreds of details or you could break some thing. All of those are things he would have loved to do, and would be able to do well. And maybe he could have come up with a system for choosing the next physical god that didn't involve "Hope the fastest guy is not a monster".
Canaletto looks awfully like the Chozo from the Metroid seriesIn fact, seeing how ancient the Chozo civilization was, one would say that Canaletto was a Chozo. A chozo who was religious/ideological fanatic who sought the “ultimate prize” to purify the galaxy in a messianic/evangelist zeal, and had his appearance changed (just like Jordan had) when he was imbued with the Avatar’s power, with which he went rampant with righteous fury.