1) Improved schedgraph support.
2) User-space lock profiling tool
3) Improved VM tree structures
4) SMP safing Giant protected filesystems.
Maybe others. I know schedgraph and the user-space lock profiling may not sound as glamorous but they have the potential to have the biggest long-term impact on performance. Please email jeff at freebsd.org if you'd like to discuss these further.
The official list of project ideas is here: