Yeah, the moon definitely holds holds a lot of relevance to the actual plot of the story in regards to poneglyphs and a possible linkage to the void century. Although I think it would be too much for him to suddenly get re-introduced in this arc, if what people said is true about the series recently crossing the half way mark, with the time skip, I could definitely see an arc where they may travel to the moon. Or more possibly, maybe Enel will get bored of ruling the moon, and based on the ruins he has visited, may realize Earth may be his new destination with the mindset of conquering it. Characters like Enel are definitely not going to disappear.