Planning a Sequel

Still no title decision for my diary novel, though I have many in mind and am trying to narrow the choices down. But just last week I began writing something to begin a sequel to the diary. I haven't gotten very far on that one, though, and have only written it in a notebook. I tend to do it this way because it's easy to do when I'm in bed, and that was when I did it 🙂

Have any of you planned a sequel for an unpublished book? How common is it to do this? My reason for wanting to do a sequel is that I had some ideas in mind for my protagonist that I think would work better as a second book, rather than trying to cram all the storylines into one book. The first book still in progress is all about the protagonist dreading getting braces. For the second one, he is dreading becoming a teenager (more to come on this). Those seem like two separate storylines that could be written and told in two books. 

I know that you need to write more than a recap of the first book when beginning a sequel, though I did begin that way and will go from there to lead into the main storyline.