You can search for popular projects that have TODO notes in the source code
To search for popular projects execute this search query for repositories.
If you judge popularity by the number of followers.
To search for the TODO notes in that repo execute this search query for code.
Because the code is indexed differently to the repository there is no direct relationship from the code to the repositories followers. So this has to be done in two steps. But it shouldn't take long to find something to work on. This could be automated in using the Github’s API but it doesn’t support the advanced search criteria.