Despite funding uncertainty, NASA is still pushing to send humans to Mars by the 2030s, and it's looking for outside input to move the Mars exploration program forwards. The Mars Program Planning Group (MPPG) is currently taking proposals for "challenge areas," which include everything from improving landing accuracy to extracting oxygen from the surface of Mars. NASA encourages anyone with experience in the relevant areas, including students and researchers outside the US, to submit papers that briefly outline how to solve any of these problems.
After the May 10th deadline for submission, NASA will pick the best entries and present them at a conference in June. The agency isn't promising funding for the projects yet; instead, it's hoping to integrate them into its mission planning program. Near-term solutions could become goals for the next program, and long-term solutions will be worked into the overall architecture of the space program. You can see the full list of challenges, along with guidelines for submissions, at the Lunar and Planetary Institute here.