This page lists tasks available for contributors. If you want to help out in Slic3r development, you could choose a task among these that are needed for sure.
See this page for a good and comprehensive page about the coding style I like.
An algorithm for Skeletonization of thin polygons. We need this for thin walls and other things. This could be the port of an existing algorithm or an original idea. A hexagonal infill pattern.
Perl bindings for the Boost.Geometry library
Logic for hollow objects [spiralize]
Better support material
GUI with preview