The thing about a cottage garden is that it "ain't 'sposed to be nuthin fancy" and that's the way it ought to be. Ideally, a cottage garden evolves over the course of several years with some plants becoming more or less naturalized. However, it's also a good idea to continually try out new plants to see how the different plants in the garden interact. It's best to include plants which bloom at different times of the year in order to have something blooming all the time. Of course, your climate will limit what will do well in your garden as will other things such as soil, pests, and shade, but there's always a plant for just about any circumstance and so you might have to do your homework in order to figure out what plants will work in your garden. Every garden is unique and so allow your garden to develop its own personality and don't try to replicate someone else's garden. Of course, there's nothing wrong with borrowing ideas and getting ideas from other gardeners, but allow your garden to unfold as naturally as possible.