Alcaraz split the four Grand Slams with Sinner in the 2024 and 2025 seasons, but injuries to the top two this year left the door open, with Zverev winning the French Open and US Open.
With Shelton reaching the Flushing Meadows final, Alcaraz expected no shortage of genuine contenders for the big titles.
"Pre-season is really important," Alcaraz told reporters after his Vienna Open triumph on Sunday.
"I can tell you that players are taking pre-season really seriously, those weeks of training hard to be in good shape.
"Sascha (Zverev) showed us what he's capable of doing. The best player of this year. He deserves to be there. Let's see if he's going to continue to do the great things he's been doing this year in 2027.
"Shelton reached his first Grand Slam final and I think he's ambitious and hungry for more next year. A couple of others also playing great and they're going to try to practice at their best to be at their best next year.
"I don't like to say it's going to be Jannik and me and the rest. Let's see who's going to be there. I'm pretty sure that a lot of players are going to show up."
Alcaraz won his seventh Grand Slam title at the Australian Open before a wrist injury kept him out for more than four months. Since returning at the US Open, he has won titles in Tokyo and Vienna.
He will look to finish the season on a high at this week's Paris Masters and the ATP Finals from November 15.
