Keep looking... for ideas, for inspiration, for things you haven't seen yet. Your job is to collect a lot of dots. The universe's job is to help you connect them.
For me, staying interested requires time away from your practice. You can't always be in it or you will actually lose interest. Not good! So go outside. Get away from it. Do other things. Sometimes you need to read. Sometimes you need to just wander around on the internet, collecting images. Sometimes it's running. Sometimes it's making art. In some ways, it actually feels more like maintenance. Like you're just doing the things you need to maintain the engine. That's part of it, I think. The maintenance is part of the whole thing running properly. It may feel like you are wasting time but being productive is not always about producing something. Production can only happen in the right conditions and maintenance is a big part of creating the right conditions. You can't force anything. Time is also a big part of it. Sometimes time is the only thing needed for it all to come together – for the dots to connect.
Also, do a lot of things. All at once. That's part of being interested. Have a lot of projects going on. Try a bunch of different stuff. The more stuff you do, the more dots you'll have to connect – you'll be able to draw inspiration, ideas, learnings from all the different projects you've worked on. Do more! All at once! All the time!