Given that The Legend of Zelda: Tears of the Kingdom is a direct sequel to Breath of the Wild, it's reasonable to assume Link will keep similar outfits to the first game. That means there's no reason for him to return to his famous green tunic, as he doesn't receive it during the main story like in previous Zelda titles, instead dressing in blue. In fact, it might be better not to go back to Link's iconic green outfit at all. It's not unusual for Zelda games to make slight changes to a few elements to set them apart. Instead of Link's tunic, there are other features players would like to see return in Zelda: Tears of the Kingdom, like bosses from previous games. In the case of Link's outfit, it might seem out of place to return to his iconic green in the sequel of a game that didn't have it as a main clothing choice.
Zelda: Breath Of The Wild Gave Link A Blue Outfit For A Reason
The decision to give Link a blue outfit over his usual green one was made with the setting of Breath of the Wild in mind. An article from Nintendo Everything contains quotes from some developers of BOTW with lead artist Yoshiyuki Oyama saying that Link wore blue early in development so that he'd stand out against the backgrounds they were designing. However, players who want to wield Zelda's powerful Master Sword in BOTW with Link's green outfit are able to obtain it through Amiibo.
Armor Options In Zelda: Tears Of The Kingdom
As in BOTW, it's likely that Zelda: Tears of the Kingdom will have armor that players can get throughout the game. Since at least part of TOTK will take place on the ground level of Hyrule, Link will be in the green background that led to him getting a blue outfit in the first place. Between the convenience of sticking out from the background and abilities to switch armor as needed, there's no reason for the green Hero's Clothes to be included.
Link not receiving his green tunic and hat in the main storyline of BOTW can seem odd to long-time players, but there are multiple armor options to choose from for those who don't like the Champion's Tunic, such as BOTW's version of the Dark Link armor. Heading to the sky in Tears of the Kingdom could make an argument for swapping the blue Champion's Tunic for a different color, but the trailers so far make it seem like the color won't pose a problem for the new background. Maybe Link should complete the classic trio of colored tunics from Ocarina of Time and wear red in The Legend of Zelda: Tears of the Kingdom. Sources: Nintendo Everything, Nintendo of America/YouTube