I wouldn't be surprised if Ultra Smash was basically just a beta for this game. They basically created the engine and HD assests to use then Nintendo rushed something out for the 2015 holiday just to have an extra game to help pad the Wii U's lineup that Fall but since they knew they had a new system on the horizon that they wanted to do better, had Camelot continue working with the engine and assets to create a much better product for the new system.
Even though that would have been kind of scummy on Nintendo's part for Ultra Smash, I hope this is the case since it means Aces has the potential to be pretty awesome since they'll have had over 2 years to develop this game using existing engine and assist, which should help give it a ton of content which has been the biggest problem with the last few Mario Tennis games. The Story mode alone give me a lot of hope.