The answers by ff524 and Anonymous Mathematician have great advice on how to evaluate a thesis topic.
As another approach, start by considering advisors that you think you may like to work with. Don't underestimate how important it is to find the right advisor. This may help you to explore what they're interested in and how it overlaps with what you're interested in.
And once you have an idea about the advisor and a topic, reach out to them! They should be more than happy to discuss potential projects with you and this will definitely help in your search for a topic that you really want to do!
Finally, I have to object to this concern in your question:
I'm sure some will be solved before I start my actual thesis
Problems in academia are rarely 'solved'. If you find a topic that you think has been solved, speak with the advisor for the project and I'm sure that they will already be considering ways to extend the topic. There is always more work to be done! Sometimes it's better not to start from scratch, although this may be discipline specific.