As of summer 2016 we are using the LAMMPS project issue tracker on GitHub for keeping track of suggested or pending new features. This includes discussions of how to best implement them, or why they would be useful.
Anyone developing a significant new feature for LAMMPS is encouraged to submit an issue about their planned implementation, especially if the feature is non-trivial to add, e.g. because it requires changes to some of the core classes of LAMMPS. This enables feedback from the LAMMPS developers regarding the planned effort and possible ideas for improvements, pitfalls, or alternatives.
Please see issue #8, issue #17, and issue #70 for examples of how to suggest code enhancements, submit proposed changes, or report related issues.
As an alternative to using GitHub, you can email the core developers.