Next year will mark Queen Elizabeth's 70th year in her role as monarch, and to celebrate, the British royal family is planning an enormous celebration. The "Platinum Jubilee" of June 2022 will include a four-day holiday weekend, a Trooping the Colour ceremony with 1,400 soldiers, and a "Platinum Party at the Palace" broadcast live. Already, there are rumors Prince Harry and Meghan Markle want to attend—Prince Harry has always been close to his grandmother, and reportedly remains so even after stepping down as a senior royal—and may even bring their children, Archie (who will be three), and their infant daughter, due this summer.
If the Sussex family does come to London next summer to honor and celebrate Harry's grandmother and Archie and his sister's great-grandmother, it's possible that they'll take part in the traditional family gathering on the Buckingham Palace balcony. "Gathering" isn't quite accurate here—technically, members of the family stand on the balcony to watch a military flyover—but it's historically been a photo op during every Trooping the Colour celebration.
Here's the entire royal family, including Meghan and Harry, in their last Trooping the Colour balcony shot, which was in 2019 (they didn't gather in 2020 due to the coronavirus pandemic):
You get the gist: All members of the royal family, even those who are not technically senior royals (you can see Princesses Beatrice and Eugenie in some of the above photos, for example), are gathered for what is at once an honoring of the military and a photo op. If Harry and Meghan do come to the U.K. for Trooping the Colour—and given Harry's great affection and respect for his grandmother, which he's made clear at every opportunity, it would be surprising if he didn't—then the whole family would likely be invited to appear on the balcony as well.
If this were to happen, it would mark Archie and his little sister's first public appearance since Archie was a newborn. Since he and his sister are growing up in California without royal titles (although they are both very much in the line of succession), they're not likely to make a public appearance until Trooping the Colour, if they do at all. Harry is expected to return to the U.K. in July 2021 for an unveiling of a statue in honor of his late mother, Princess Diana, but he'll likely go alone; the trip is expected to be just weeks after Meghan is due to give birth to their daughter.