She might be shocked at first. Given that in the original story Morgana has her own complex relationship with magic, seeing Merlin's magic could stir a mix of emotions in her, like envy or a sense of relief that she's not alone in her magical abilities.
One idea could be the characters from Persona 4 and Persona 5 swapping their worlds. So the Phantom Thieves end up in Inaba and have to deal with the Midnight Channel mystery. And the Investigation Team finds themselves in Tokyo and gets involved in the Metaverse. It would be really cool to see how they interact with the different environments and characters.
The popularity of 'persona 4 pregnant fanfiction' is likely not very high in the overall 'Persona 4' fanfiction landscape. The 'Persona 4' franchise is known for its unique blend of mystery, high school life, and supernatural elements. While fanfiction allows for a wide range of creativity, pregnancy - themed stories might not appeal to the majority of fans who are more interested in the game's established themes such as the investigation of the Midnight Channel, the growth of the Social Links, and the battles against Shadows.