Is Aang and onji from the show Avatar Last Airbender are going to be couples in future?

Aang and Onji are not going to be a couple in the future of *Avatar: The Last Airbender* because Onji never existed in the show.

It's likely you're thinking of Katara, who is Aang's love interest and eventual wife in the series.

Perhaps you misremembered the name or are thinking of a different character.