Super Smash Bros. Ultimate has one unidentified character left for the Fighters Pass, as well as an undetermined number of DLC characters who will appear in the future. It's possible that a cryptic message on Twitter might have revealed the identity of the final Fighters Pass character or a later choice for a DLC slot.
The speculation started when voice actor Scott Menville uploaded a post to Twitter that talked about a cool voice over project that he was working on but was unable to discuss due to signing an NDA.
Scott Menville has worked on a number of cartoons over the years, including ones based on DC and Marvel properties. Menville has also worked on some video games and it's for this reason that fans believe the secret project is Super Smash Bros. Ultimate.
So, who is the identity of the potential character? One of the characters that Menville has voiced in the past is Lloyd Irving from Tales of Symphonia, which started out as a Nintendo GameCube exclusive title, before being ported to other systems a year later.
Tales of Symphonia might seem like a bizarre choice for a Super Smash Bros. Ultimate representative, but Lloyd could be representing the series as a whole, in the way that Joker represents the Shin Megami Tensei series as well as Persona. Tales of Symphonia was a huge deal in Japan, leading to a sequel on the Nintendo Wii and several books, manga, and an OVA series. Lloyd's addition to the game would mostly appeal to the Japanese fanbase and his inclusion would likely draw ire from the international fans, especially as the Tales series isn't as popular as other RPG franchises in the west.
It's possible that Menville's secret project is unrelated to Super Smash Bros. Ultimate or video games itself, so nothing is set in stone at the moment. The fact that Banjo-Kazooie and Terry Bogard made their way into Super Smash Bros. Ultimate means that Nintendo isn't concerned about mainstream popularity for the DLC choices in the game, so it's entirely possible that a Tales representative could make their way in.
Source: Scott Menville/Twitter