I'm waiting patiently for full 3rd party Audio Units plugin support, but I think the following would be useful:
- DSP tools like the ones provided in JRiver Media Center - especially parametric EQ
- Fixed volume level (so it can be set once and it won't accidentally change) Useful for when using EQ to make >+0db adjustments
- Metering Visualizations
- Insert silence when automatically switching sample rates - prevent nasty pops