I don't really see how the ending was fanservice. The Vader scene was about 30 seconds long and not nearly as long or drawn out as it could have been, his role seemed in character for him. As for the rest of the battle, a lot of fan-favorite vehicles and whatnot do appear, but it makes sense for the time period, that's like complaining that WW2 vehicles are fanservice in a movie set during WW2. I've just never understood why people complain that Rogue One has too much fanservice. Aside from Ponda Baba and Dr. Evazan, everything felt appropriate, the film even made attempts at adding new vehicles and tech. Bringing X-wings and Tie Fighters back 30 years later for TFA was the pure definition of fanservice.