Like the double-sided lightsaber mentioned earlier, we do see a battle droid in the background of Babu Frik’s droid shop in the final Rise of Skywalker trailer. That was very cool to see, but battle droids need to be more than an Easter egg in Episode IX. They need to have a purpose in the film.
There is a Droid Control Ship spotted in the ramshackle armada of Resistant fighters in the final trailer. It is safe to assume that the ship would also have an army of battle droids.
The Resistance is clearly massively outnumbered and having a fleet or two of old battle droids would be a quick and easy way to even the odds. If this droid hacker Babu Frik does align himself with the Resistance in The Rise of Skywalker, maybe he could figure out a way to reprogram the droids so they attack the first order or at least obey the commands he sends them via the ship.
Fans all over the world would love to see Battle Droids being used against the First Order and hear one final “Roger Roger”. It would be poetic justice to see this group of droids help defeat Palpatine’s army in the end, when they were used to help start his reign in the first place.