Both GRASS and pgRouting project participate in GSOC.
Feel free to introduce yourself in those developer categories if you find those projects interesting.
@neteler did a nice writeup of how to contribute to GRASS here - Seeking Guidance for Contributing to GRASS Development
You can see videos of GRASS from prior conferences - GRASS GIS - OSGeo which might help you with ideas
As for pgRouting take a look at what students have done in the past for ideas - Google Summer of Code · pgRouting/pgrouting Wiki · GitHub
Videos from conferences and also @cvvergara giving developer instruction around the code base - pgRouting - OSGeo
I would suggest looking at these two videos @cvvergara did during GSOC office hours -