Or you can follow the directions here:
http://manual.calibre-ebook.com/develop.html to set up a development environment and hack the code. calibre is written in python.
As to what needs doing -- whatever you personally would like to see calibre support is a good start.
chaley created plugboards and various infrastructure around templates too I think -- because it fulfilled his need and he wanted to see it happen.
Alternatively, browse the calibre forums here to see what people are interested in.
Offhand I can think of a few -- proper annotations support in the viewer, client and multi-user mode for the library, which should resolve peoples' issues with networked libraries, macros/function mode in the editor...