The family trees of the X-Men are often among the most complex in the Marvel Universe, with unexpected branches tying together unlikely corners of the universe to one another. This includes a child of a classic X-Man that's been in the background for quite some time and could finally be due for a return.
During his early tenure with the X-Men, Colossus had a fling with a woman from the Savage Land which resulted in Colossus having a son he's never truly known, and the current events in the X-corner of the Marvel Universe might have perfectly set up their reunion.
In 1978's X-Men #114 by Chris Claremont and John Byrne, the X-Men found themselves separated after a battle with Magneto. Cyclops, Wolverine, Storm, Colossus, Nightcrawler and Banshee were believed dead by the rest of the world, but they had really escaped into the Savage Land. While there, they discovered unlikely allies with the Fall People. Among them was a woman named Nereel, who quickly took an interest in Colossus, a plot point that was also touched upon in Classic X-Men #21 by Claremont and John Bolton. After spending a night together, Colossus and the rest of the X-Men were drawn away into a conflict with Sauron and then Garokk. However, Colossus was left unaware that his encounter with Nereel had left her pregnant with his child.
Neerel reappeared years later in X-Men Annual #12 by Claremont and Art Adams. A tie-in to the "Evolutionary War" crossover and coming during the Outback Era of the team -- when the X-Men were believed by the world at large and consisted of Storm, Wolverine, Colossus, Rogue, Psylocke, Havok, Longshot, and Dazzler -- the story saw the group return to the Savage Land. Teleporting there thanks to Gateway, they discover the village of the Fell People decimated by Terminus. Coming up against Garokk once more, the X-Men fight him and help protect the remnants of the Fell People, including Neerel and her son Peter. Colossus remained blissfully ignorant of the truth to young Peter's father and eventually left with the X-Men when Neerel was made chief of the Fell People.
Peter has shown up a handful of times since then, often in brief shots that confirm his continued survival. It's never been confirmed whether or not Peter is a mutant like his father. However, circumstances might have been perfectly positioned for Peter to finally meet his father. The Savage Land has become a fertile place for the mutants of Krakoa, who use the Savage Land as a place to grow many of the drugs they use to help heal the sick across the globe. Among the most prominent of these farmers is Colossus, who found a place amongst them after having a falling out with X-Force over their increasingly dark duties.
This means that more often than not, Colossus is secretly near the son he never knew he had. If the two encountered each other, enough time might have passed for their resemblance (or potentially even similar powers) might finally clue him in on the truth of his parentage. It also comes at a time when Colossus has both been elevated within Krakoa society (thanks to his recent ascension to the Quiet Council) as well as quietly been betraying the nation, thanks to Mikhail Rasputain's use of a "writer" to control Colossus into his service. Having an unexpected son could only complicate the situation further. Peter could prove to be a wildcard in his father's saga, or he could venture into Krakoa himself to learn more and about the mutant birthright he may have within him.