Pick an open source project that you like, fix a couple of bugs to show you're serious then talk with the project maintainer(s) and tell them you want to do something that takes 150 hours or so.
The reason for doing something open source is that you can point to it when a future employer asks. If you choose a "brand name" project, you can get a lot of kudos for being associated with something like that.
One way is just go into the community of the open source project you're interested in and ask for projects ideas.
Also searching for "project name" AND "project ideas" could sometimes lead to good results. for example [1].But of course before choosing a project in such way , it's better to understand a bit more about complexity and scope.
The reason for doing something open source is that you can point to it when a future employer asks. If you choose a "brand name" project, you can get a lot of kudos for being associated with something like that.