A possible plot is that they find out they're having twins. This adds an extra layer of excitement and worry. They start preparing two of everything, from baby clothes to cribs. Natsu might start thinking of all the fun things he'll do with his two kids in the future, like teaching them fire magic. Meanwhile, Erza is busy making sure they have a proper home environment for the babies. The guild could also hold a big celebration for the upcoming twins.
Well, a plot could revolve around the challenges they face during the pregnancy. For example, Erza might still want to take on difficult missions, but Natsu is worried about her and the baby's safety. So they have to find a balance between their adventurous lives and the new responsibility of having a child. Also, they could discover new magical abilities related to the baby growing inside Erza.
One idea could be exploring how their guild mates react to the news. Some might be overjoyed and offer all kinds of support, while others could be a bit shocked at first. Another could be focusing on how Natsu and Erza themselves adjust to the idea of becoming parents. Natsu might be a bit clumsy at first but full of enthusiasm, and Erza could be more practical and start planning everything right away.