Now no, this won't have Tom Selleck as the titular star, Thomas Magnum, but will instead focus on his daughter Lily 'Tommy' Magnum. According to the article, her career with Naval Intelligence, having followed in her father's foot steps, was deep-sixed and she is returning to Hawaii to take up her father's PI firm instead.
https://www.cnet.com/news/magnum-p-i-sequel-coming-to-abc/
It's still early, and so there's no telling if any original cast will be making appearances, but I have have high hopes for this, as this show was one of my favorites as a kid. Thomas Magnum, the perpetual child-like bachelor solving crimes from the seat of 'his' Ferrari, with his cadre of Army buddies and the 'is he or isn't he Robin Masters,' Higgins. It can't do itself wrong by going right back to the formula that made that show so great.
As the article says, there's no word as to whether Tom Selleck will be making a cameo, which really should happen! Selleck does have another show he's doing, but a few guest appearances would be a good thing.