Clone Troopers
Attack of the Clones introduces one of the biggest elements of the Prequels and really the overall Star Wars saga, and that is the clones. The clones of Jango Fett play a huge part in the downfall of the Jedi and the rise of Emperor Palpatine and the Empire.
Darth Sidious wisely used these troopers and their impressive arsenal of vehicles and other weaponry to fight alongside the Jedi. For three years these clones fought valiantly side by side with the Jedi until that evil menace executed Order 66. The clones then turned on their Jedi allies and murdered nearly all of the guardians of peace and justice in the galaxy.
It wouldn’t be possible to see an actual clone in The Rise of Skywalker. With the time that has passed since the Clone Wars and the start of The Rise of Skywalker an actual clone would be nearly 80 years old. No one wants to see a decrepit old clone lumbering around.
A better way to include a clone in the final film would be to have the crew stumble on to some armor. Perhaps they find it hanging in an old ship, or it is shown slightly buried but visible, on some old battleground that the gang is wandering across. Even just a quick Easter egg would be enough to tie in this all-important part of Star Wars lore in the final film.