business-oriented plugins would be great, but if you're going to be part of the GSoC, it'd probably be better to work on core Chili code.
Maybe pick up a few of the most desired (and/or oldest) bugs in the issue list - not forgetting to draw inspiration from the 'must-have' issues that are 3+ years old in RedMine's issue tracker, stuff like multiple repository support, Private Issues, export to PDF/Word, assign issues to groups, and similar.
Personally, I think adding better reporting options to the trackers would be a 'sales' benefit to Chili as that's what my managers always talk about when discussing bug trackers (as if that's the only important thing, sigh)
There are already invoicing and time-tracking plugins available, so re-inventing these might not be worth the effort, not when you could be a hero by implementing multiple repositories, automatic ticket duplication for all open versions, and tracker report and graphing :)