A Story Mode is the biggest reveal, but Waluigi’s voluptuous hair is a close second.
After Mario Tennis: Ultra Smash, it's reasonable to doubt Camelot's sports game prowess. The once-legendary developer has had far more misses than hits. While Mario Golf: World Tour on 3DS was largely excellent, Ultra Smash and Mario Tennis Open were varying degrees of disappointing. Even their contribution to Mario Sports Superstars wasn't great.
Mario Tennis Aces, on the other hand, shows a lot of promise in the brief look during the January Nintendo Direct Mini. The core of that promise is in the featured Story Mode, which as Nintendo notes, is the return of that kind of deeper tennis single-player mode for the first time since the Game Boy Advance. From the little we see of Story Mode, it doesn't seem to resemble Mario Tennis: Power Tour Advance's RPG-esque story in the slightest.
Story Mode, as shown here, puts players in control of Mario, who appears to travel a world map to go to different areas, competing in matches, challenging mini-games, and fighting bosses. The former two are standard Mario sports fare: Mario plays tennis and does some sort of Piranha plant volley mini-game. Bosses have appeared in past tennis outings, but their use here seems to be a little more distinctive than, for example, their appearance in Ultra Smash. The bosses there were just matches against a large competitor. The Petey Piranha boss battle shown here looks to be a little less stringent of a tennis match. Aside from Petey screaming and swatting tennis balls back at Mario with his arm (is that legal?), we don't see much, though.
The game takes place on some island, with Mario and company playing in a large stadium. That appears to be where the story starts, and from there, Mario goes off to other parts of the island. The first area, which is shown here, looks to be a forest area. Donkey Kong's in it. Piranha plants are there, too. As video game law dictates, other parts of the island likely house different environments. Maybe there's a volcano at the top? An ice area?
Outside of Story Mode, this trailer doesn't show much. The playable characters featured are Mario, Luigi, Wario, Waluigi, and Donkey Kong. All of the humanoid characters wear visors instead of their typical hats. It's weird; Waluigi has quite the voluminous and bouncy hair whereas Wario might just be wearing a hair helmet.
The gameplay looks to be reminiscent of past Mario Tennis games, which was always the strong part of even the weakest Mario Tennis games. The only apparent twist seems to be some of sort power meter for each player. Present in the upper corners, this meter looks to build up during play, likely resulting in some sort of power-up. The only glimpse of what it might do can be found in one of the Story Mode parts as Mario is shown to be "powered up" and hitting the ball harder.
Mario Tennis Aces will be out on Switch in the spring, so speculation for this game might not last too long. Hopefully the early promise holds true for the final game.