I don't know DSP at all. An incremental FFT is the core tool for building analog-domain effects. A simple room reverb effect that does one sound bounce instead of feeding back would also be handy, and an easy coding project. Loop support for all formats that know about it. Someone needs to add this in.
Some file formats have space for embedded comments.
These are currently thrown away. Printing them out, carrying them forward, and an to add new ones would be handy. SOX includes skeleton format files to assist you in supporting new formats, sound effect loops, and special-purpose programs. The full skeleton format, skel.c, helps you write a driver for a new format which has data structures. Skeleffect.c is a starting point for writing a sound effect loop.