Though Tekken producer Katsuhiro Harada initially seemed against the idea of guest characters, noting that Akuma fit the series however, I suppose once you open the door it's hard to close it back up again.
Bandai Namco announced their slate of planned DLC after Tekken 7's June 2nd launch, and it notably includes two "exclusive playable guest character[s] from other videogames' licenses." First there's a DLC batch coming this Summer with a new game mode and 50 new costumes, then a batch Winter 2017 (featuring one guest character with a special stage and costumes), and Spring 2018 with the final guest character, according to the press release.
Each of the DLC releases will be available for purchase individually, but will also be covered by the season pass (folks who purchase the season pass will be treated to 35 metallic costumes as well). I'm personally in favor of fighting games crossing over like this (RIP Tekken X Street Fighter, we miss you everyday), so what could hurt from seeing a familiar character in a new style?
What guest characters would you like to see duke it out in Tekken 7?