One possible plot could be that the Marauders, in some magical time - travel or alternate reality scenario, get to meet baby Harry. They might be overjoyed to see him and immediately vow to protect him. Sirius could be especially attached, given his future relationship with Harry. They might also play with baby Harry, making him giggle with their magic tricks.
In the 'the marauders meet baby harry fanfiction', it could be that the Marauders meet baby Harry due to a spell gone wrong. The spell accidentally transports them to the time when Harry is a baby. At first, they are confused, but then they realize the significance of the moment. James might hold Harry and tell him stories of his future adventures, while Remus and Peter look on fondly. They could also try to give some advice to baby Harry in a magical way that he would unconsciously remember later in life.
Well, perhaps in this fanfiction, the Marauders meet baby Harry when they are spirits. They are unable to interact fully with the physical world but can still watch over him. They are filled with a sense of pride and sadness at the same time. Pride because of who Harry is destined to be, and sadness because they know the hardships he will face. They might try to send positive vibes or small magical nudges to help him in his early days.