"The Umbrella Academy" Season 2 finale teased that it is not the last fans will see of Harlan after it revealed that he still has superpowers. He was able to levitate objects and a fan theory claimed that he could return in Season 3 as an old man or as the same age.
Vanya did not know that Harlan still has a bit of her powers left in him. She thought that after she absorbed them back in, the autistic boy would be safe from worrying about how to control his powers.
However, the Season 2 finale definitely teased the contrary and according to CBR, Harlan may find this power helpful. It could make him stay young forever (just like Five) and he could even exist in the new 2019 that the Hargreeves siblings accidentally created with their time travel.
It is possible that Harlan could be at the same age as he was in Season 2, and the powers helped him stay young. If not, he could already be in his seventies and still has his powers when he appears in "The Umbrella Academy" Season 3. Either way, Vanya is responsible for his existence in the new 2019. After all, she unknowingly gave him her powers when she saved him from drowning after she performed CPR on him.
Harlan's possible existence in the Hargreeves siblings' modern world could also be because of his connection with Vanya. The powers aside, he has formed a close bond with her during her stay at his home in 1963.
Fans may remember that Vanya acted as his babysitter. She knew how to calm him down with music from the turntable. Vanya found a family in Harlan and his mother after they decided to let her stay at their home because she had amnesia and she was trying to track her family down.
That tease in "The Umbrella Academy" Season 2 finale was about how his storyline will continue in Season 3. If Harlan does appear in the installment, then he might be a good accomplice for the Hargreeves siblings as they tackle a new, more powerful enemy, The Sparrow Academy. Vanya is the strongest among her siblings and since she shared her powers to the young boy, then it means he too shares her strength.