A similar situation happened to me. I write a series. I've written it for a very very long time. I was always told that for a series to survive, for it to be picked up and liked each story had to stand alone. So that's what i did. Each book was a new story, what i thought made it a series, was the same characters. the way you watch they grow up, there abilities morph and enhance, there bond together strengthen.
Then i watched the last Harry Potter movie. I'm not a fan of Harry Potter, never was, never will be, but my boyfriend was and drug me to see it, like he did for the last three of those movies. As i was watching it i could hear kids and adults alike crying over it. For some reason this bothered me. I wanted to know what made them cry over the ending of this series, what made this last movie and book "SO EPIC" as everyone told me. Some told me it was the characters, some told me it was simply because it was the end. Then i asked my boyfriend. He simply said "it so amazing because its a battle you watch/read for years and finally the side you pray and wish to win does. Everything is put to rest and its a peaceful epic ending."
It all got me thinking. Like really thinking about where my series was going. it didnt have that OMG epic ending. The last book was just going to bring up some old characters previously thought to be died, and end it all. I realized the whole series was just a gigantic let down.... it didn't end with a band at all. I sat there thinking that if my series were to be made into movies, not a single person would feel the same way towards my ending as they did to Harry Potter.
One day driving home from work it was really bugging me. I felt like i just couldnt pull off my series now because there was this huge problem with the stupid minimal ending. I dont really know how the thought came to me... Honestly i think it was always there i just thought it was to drastic. TOO much of an "OMG NO WAY" kind of deal. That it wold throw the readers through too much of a loop, but the more i thought of this concept the more i realized. I didnt need to re-write a single thing. All the slight plot holes that no one would even think were plot holes but just bugged me, would be instantly solved, every single question revolving around the entire series answered. Everything would fall into place so perfectly it was shocking. All i would have to do was add one last book (or split it into two, havent decided yet) that would bring the ENTIRE series to a full circle epic ending.
My boyfriend who is my only fan and the only person who knows every aspect of my series, i thought wouldnt like this new idea. I thought he would think it was stupid but i told him anyway... told him i was considering it. We were driving and after i told him he was really quiet. I looked over at him and his jaw was just dropped as he stared ahead. Then all he could say for the next 10 minutes was "Oh my God, Oh my God!" Low and behold he loved it even more then me. SO now problem solved.
Makes me now wonder why I didn't come up with the idea sooner.