I would try to analyze and solve one or more of the problems published on Kaggle Competitions. Note that the competitions are grouped by their expected complexity, from
101 (bottom of the list) to
Featured (top of the list). A color-coded vertical band is a visual guideline for grouping. You can assess time you could spend on a project by adjusting the expected length of corresponding competition, based on your skills and experience.
A number of data science project ideas can be found by browsing
If you have skills and desire to work on a real data science project, focused on social impacts, visit
DataKind projects page. More projects with social impacts focus can be found at
Data Science for Social Good webpage.
Science Project Ideas page at
My NASA Data site looks like another place to visit for inspiration.
If you would like to use open data, this long list of applications on
Data.gov can provide you with some interesting data science project ideas.