Guinan (Whoopi Goldberg) could appear in Star Trek: Picard season 2, and there are a few ways this could happen. When Patrick Stewart appeared on The View to promote his CBS All-Access series that charts the next chapter of Jean-Luc Picard's life, the iconic actor said to Goldberg: "Alex Kurtzman... and all of his colleagues, of which I am one, want to invite you into the second season." If Goldberg accepts, Guinan would follow in the footsteps of her fellow Star Trek: The Next Generation alums Jonathan Frakes, Marina Sirtis, and Brent Spiner, who all made memorable appearances in Star Trek: Picard season 1.
Guinan was introduced in TNG season 2 and she appeared in 29 episodes as well as in two TNG feature films. The mysterious alien joined the U.S.S. Enterprise-D at Picard's personal request and she served as the bartender of Ten Forward. As an El-Aurian, one of a race of "listeners," Guinan is evidently hundreds of years old; in fact, she first met Picard in 19th century San Francisco when the Enterprise crew time traveled in the "Time's Arrow" two-part episode. Aspects of Guinan's extremely-long and enigmatic life have been teased by her rivalry with Q (John de Lancie), who called her an "imp" and suggested she has gone by other names in the past. Regardless, Guinan has proven herself to be a valued and impactful confidant to Picard and the Enterprise crew. Guinan also survived the destruction of the Enterprise-D in Star Trek Generations since she attended Will Riker and Deanna Troi's wedding in Star Trek: Nemesis, which was the kindly El-Aurian's last canonical appearance.
Since she is very likely still alive, Guinan could easily appear in Star Trek: Picard season 2. It hasn't been made clear if Guinan ever joined the U.S.S. Enterprise-E in her original capacity as the ship's bartender, but regardless, the wily El-Aurian may have set up a new establishment or found a new vocation elsewhere, possibly even on Earth. In fact, Guinan is the perfect person for Picard to turn to now that he has a synthetic body after the Admiral's death and resurrection in Star Trek: Picard's season 1 finale. Just like how Jean-Luc's old colleague from the U.S.S. Stargazer, Dr. Moritz Benayoun (David Paymer) paid a visit to Picard, it would be a simple matter for Guinan to drop by Jean-Luc's vineyard in La Barre, France to counsel her old friend on how to cope with his new android body and anything else that might be troubling the Admiral's mind, heart, and soul.
There's been no hint yet of Star Trek: Picard's season 2 plot, but since Jean-Luc will keep his new crew aboard the starship La Sirena together, Guinan could also be one of the people they encounter in their travels. Guinan took a special interest in Commander Data when they were on the Enterprise together and she could tell Soji (Isa Briones) more about her father or help her continue to deal with her trauma after she learned she was synthetic. Additionally, Guinan's advice could also be a big help to Raffi Musiker (Michelle Hurd), who turned to substance abuse after she was disowned by her ex-husband and son.
Another way for Guinan to appear would be through the Nexus, although this would be a longshot considering how unpopular Star Trek Generations' space ribbon was among Trekkers. Still, the film established that an "echo" of Guinan remains in the Nexus and it would be quite bold of Star Trek: Picard's creative team if they wanted to include the Nexus in season 2 in order to possibly redeem that underdeveloped concept.
One hurdle regarding Whoopi Goldberg appearing in Star Trek: Picard season 2 is that the actress has aged since her last appearance. There would either need to be de-aging CGI used for Goldberg since Guinan ages very slowly. But it's also possible Guinan's aging could be part of her storyline and this could cleverly open up a way for Jean-Luc Picard to help his old friend the way she helped him numerous times on Star Trek: The Next Generation.