I think Baby is a really interesting character to include. His ability to possess others and his thirst for power create a lot of drama. Also, Super 17 has a unique design and power set that could be explored more in a fanfiction. And let's not forget about Omega Shenron. He is the ultimate villain in Dragon Ball GT and his final battle with Goku could be re - imagined in all sorts of ways in a remastered fanfiction. You could also include some of the minor characters like Uub and show how they develop further in this new version of the story.
Well, aside from the obvious main characters like Goku, Vegeta, and Pan, I would say the Supreme Kai of Time could be an interesting addition. She has the power to manipulate time, which could add a whole new layer of complexity to the story. Another character could be Pilaf and his gang. They could be involved in some sort of comical sub - plot that also ties into the main story. And for a more unexpected character, Broly could be introduced into the GT Remastered universe. His raw power and his backstory could create some epic battles and interactions with the existing characters.
Goku is always a must - include. His never - ending quest for stronger opponents and his pure - hearted nature make him a central figure. Pan is also very interesting as she represents the new generation of Saiyans. And of course, Vegeta, with his pride and his own journey of growth throughout the series.