While the character is definitely missed, it would be a mistake for Carl Weathers' Apollo Creed to return in a future Rocky movie. Sylvester Stallone famously penned the first draft of Rocky in 3 days when he was a struggling actor and writer. He was inspired to write the ultimate underdog story after watching a boxing match between Muhammad Ali and Chuck Wepner, with the latter managing to last a full fifteen rounds before being knocked out.
While producers tried to buy the script from Stallone and offer it to bigger names, like Robert Redford (The Old Man & The Gun) or James Caan, he refused to sell it without playing the lead too. The surprise success of Rocky led to a franchise that's still running over forty years later, with Creed 2 being the most recent installment. Stallone would maintain a heavy creative hand in each entry too, including writing and directing most of the sequels. The star appears to have retired from the part, however, declaring Rocky Balboa retired following Creed 2, with the franchise now belonging to Michael B. Jordan's Adonis Creed.
One of the most shocking, defining moments of the Rocky franchise came with 1985's Rocky IV, where Rocky's former rival turned best friend Apollo Creed (Carl Weathers) is killed in the ring by Russian boxer Ivan Drago. Rocky's failure to stop the match and save his friend would haunt him for decades to come, particularly when he finds himself training Creed's son Adonis years later. Despite the character having died in the fourth entry, Apollo continues to cast a big shadow over the series. From Rocky's lingering regret to Adonis having to step out from his shadow and forge his own legend, Apollo's very much still a part of the franchise.
One of the tragedies of the Creed series is that Adonis never got to meet his father, and considering his death it would be difficult to contrive a good reason for Weathers and Jordan to share a scene. The closest fans have come is a sequence at the end of Creed 2 where Adonis visits his father's grave, after defeating Viktor Drago, the son of Ivan. That said, Ivan's performer Dolph Lundgren (Aquaman) revealed an early draft of the script featured the return of Apollo Creed in ghostly form. Unfortunately, Lundgren didn't elaborate on Apollo's planned role in the story or who he would have appeared to.
While Carl Weathers was undoubtedly fantastic in the part, and fans might welcome a cameo, it would be a mistake to bring Apollo back. It's not surprising to learn it was considered, but as Lundgren himself stated, it would just feel "hokey." The Rocky franchise has (mostly) grounded itself in a realistic world, so having Apollo appear as a ghost or hallucination would be out of place. He could potentially appear in a flashback, with Weathers de-aged, but this wouldn't be satisfying either, considering he couldn't share a scene with Jordan.
Apollo Creed's death had a major, weighty impact on the Rocky franchise, and the movies have never tried to lessen that tragedy. Maybe there is a way to bring Carl Weathers back that is both emotional and makes sense within the story, but it's difficult to see how it could be done. With Sylvester Stallone seemingly stepping aside and Adonis finally making peace with his father's legacy, its time for the series to focus on Adonis moving forward. An Apollo cameo would risk being a fan-pleasing gimmick at this stage, instead of something that really needs to happen.