Agreed. From what I've heard, it's better to pace yourself and try to sell your first novel as a "stand-alone MG novel with series potential," especially if this is your first book. Publishers will base series potential on how well your first novel sells, so your work really needs to stand alone and speak for itself.
BTW, I am sooo jealous that you've already got so many big ideas... I just started the second book in my (hopeful) two to three part series, and I'm already wondering how exactly I'll fill up the next books... Hahaha, I'm hoping they magically fill themselves in like my first book did. Best of luck to you!