As noted, a new promo for a mystery person aired during last night's WWE NXT episode, teasing a return to the brand at the upcoming "Takeover: 31" event on Sunday, October 4. You can see the vignette above.
The mystery person disguised their voice, so while it can be either a male or a female Superstar, based on former NXT Women's Champions, it looks like this will be the return of a male wrestler. WWE's video description referred to the mystery person as an "unknown insurgent" who has "declared they're coming back" to NXT "to take what's theirs."
The main name being speculated on social media seems to be Bo Dallas. Dallas, who is still a member of the SmackDown roster, was the third NXT Champion in history and held the title for 260 days. He has not wrestled since WWE Crown Jewel 2019 on October 31, when he and Curtis Axel lost a Tag Team Turmoil match to The New Day.
Other names being speculated on include Kevin Owens, Samoa Joe, and Bobby Roode, among others. Roode has been out of action for several months due to COVID-19 travel restrictions, but it was reported last week that he is expected to be back within the next week or so, but likely on the RAW brand.
For those who missed it, last night's promo included a line about returning to the brand.
"I'm not sure what happened. I helped create something out of my own blood, years and years of dedication... to be taken over by paper champions. These NXT championships have been in isolation for far too long, now I've come back," the mystery person said as their character was shown breaking protective glass on the video to retrieve a title belt. "The Takeover is mine."
Stay tuned for updates on the mystery vignette.