Star Wars Jedi: Fallen Order did itself a service by not focusing on any of the more well-known Star Wars characters in its first installment. Some of these individuals do make appearances in the game, but their roles are either very minor or just Easter eggs. They don't impact the overall story much, if at all. Because Fallen Order is set up like this, its original cast of characters has the luxury of integrating themselves into the Star Wars universe without fear of having attention taken away from them.
However, now that Cal Kestis and his fellow Mantis crewmates have introduced themselves, Star Wars Jedi: Fallen Order's sequel can be a little more flexible with how it includes the Star Wars universes more popular characters. In fact, more well-known characters should appear in the sequel than in the first game. Luckily, Fallen Order takes place during the "Dark Times" era, where several popular characters exist to take part in Fallen Order 2's story. None of them should be mainstays, of course, but their involvement can be more significant than what it would be in the first title. Minor spoilers ahead.
Ashoka Tano
Ashoka Tano might be the best character Star Wars has for introducing new characters into the greater story. She's both popular and one of the very few characters that lived before and after the Galactic Empire, making it possible for her to interact with people from all three major Star Wars trilogies and their time periods.
During the Dark Times, Ashoka helped establish the Rebel Alliance by rallying several different smaller rebel cells to the cause. Since the Mantis crew basically becomes such a cell by the end of the first Fallen Order, the sequel could fit Ashoka in perfectly by having her give the Mantis direction on how to fight the Empire. Even better, Ashoka could do this while still maintaining a cameo role. She would interact with the crew from afar using her anonymous identity "Fulcrum," which she created upon becoming head of the Rebel Alliance's intelligence network. This job was a sensitive one, and the Empire already saw Ashoka as a high-value target for being Anakin Skywalker's apprentice, so the extra anonymity granted by Fulcrum was necessary.
Kanan Jarrus
Kanan Jarrus is a lot like Cal Kestis. The two of them were young Padawans when Order 66 first took effect, and both of their masters died protecting them. They both then struggled with a sense of guilt over not being strong or brave enough to help their teachers when they needed them most.
Cal recovers from his traumas and becomes a Jedi Knight a lot sooner than Kanan, but it would still be interesting to see how the two interact. Maybe Cal can be one reason Kanan decides to fight back against the Empire when he later meets Hera Syndulla. How the two would interact with each other would, of course, depend on precisely when Star Wars Jedi: Fallen Order 2 takes place on the timeline. It still has 14 years left to explore before A New Hope and 11 before the Galactic Civil War era, so Fallen Order 2 probably won't jump too far into the future. With that in mind, Cal would likely interact with the Kanan that was a smuggler and thief. He would be the polar opposite of what Cal is as a Jedi.
Hera Syndulla
Speaking of Kanan, Hera is another character that would fit in nicely with Fallen Order 2. Aside from showing the Mantis' pilot Greez a thing or two about flying, Fallen Order 2 could use Hera the same way it used Saw Gerrera.
Saw led multiple head-on attacks against the Empire, and in the first Fallen Order, Cal assisted him with freeing Wookie slaves on Kashyyyk. Hera's a lot less hot-headed than Saw, but could still play the same role in the second game. The Mantis could help her with something like a relief mission, and be what gives her the idea to start a crew of her own on her ship, the Ghost. Hera's already making an appearance in Star Wars: Squadrons, so there's no reason she can't make one in Fallen Order 2 as well.
Darth Maul
During the Clone Wars, Darth Maul took over and unified several different high-profile crime families and dubbed the new organization "The Shadow Collective." But while the Shadow Collective came about during the Clone Wars, it was most active during the Dark Times, meaning Cal and his crew can butt heads with it in Fallen Order 2.
The first Star Wars Jedi: Fallen Order's story established that Greez has a history of getting into trouble with the underworld. He owed a debt to the Haxion Brood's Sorc Tormo, and not paying it led to Cal getting kidnapped and forced to fight for his survival. Fallen Order 2 could make it so that Greez's past extends to other underworld families, including some under Maul's control, which would create opportunities for the Dark Side-user to make cameo appearances.
Darth Sidious
The appearance of Darth Sidious/Emperor Palpatine in Fallen Order 2 should be a given. Darth Vader has already shown up in the first game, so it only makes sense that his master would get some screentime in the sequel. Sidious could even play the same role Vader did in Fallen, acting as a force of nature that can only be run from, not fought.
Darth Vader's cameo happens after defeating Fallen Order's final boss, and it's literally just him showing off how powerful he is. Using the force, Vader tears apart his surroundings with little to no effort, and without hesitation, kills pretty much anything that gets in his way, including his own men. All Cal can do is run and pray, as any attempt to fight Vader head-on just immediately ends with Cal having his neck snapped through the Force Choke technique.
Fallen Order 2 could do something similar with Darth Sidious. Cal could meet him toward the end of the game and be forced to retreat because Sideous is just as overwhelming an opponent as Darth Vader. If Cal doesn't escape Sidious in time, the Dark Lord of Sith would kill him through Force Lightning. Chances are, though, the developer would be looking to show off Sidious in an all new light, meaning he would appear in some unexpected way but as powerful as ever.
Star Wars Jedi: Fallen Order is on PC, PS4, and Xbox One.