One of the things you can do to keep your interest and motivation high, when you have no idea what to photograph, is begin a personal project. A personal project is set by you, so it can have any specific rules you set or no rules. It can be about colors or someone/something you're passionate about (family/hobby/charity) or timed (photo a day/each January) or new skill (macro/abstract) or...or.... You get the drift, right? You set your own parameters.
The intent is to find something that will hold your interest, regardless of what you normally photograph. So, when you are in a slump or tired of shooting what you always shoot, you do some work on your personal project to re-ignite that spark of excitement. You can do the work on cold, rainy weekends when you don't want to venture out. You can have several at once: I love to restore old photographs, especially for my genealogy habit; and I love to shoot for certain charities. Lately, I've been enjoying Civil War sesquicentennial photographs. It blends my desire to get out of the house, with history and people photography – all things I love to spend time with.
Professional photographers often use personal projects to give them variety from what they shoot for money. If you're known as a wedding photographer, your clients probably won't be interested in seeing your abstract photography, no matter how good it may be. Personal projects can act as a release valve for them, without the pressure of submission deadlines or budget worries. These projects are usually started with no expectation of making money from the work; it's just something to enjoy the photography. It helps improve their work in the genre of photography for which they get paid.
Consider starting a personal project of your own. You'll keep your interest level high and learn more. You can practice new techniques and keep things fun. It works for me.