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. Especially if a planned or proposed feature is non-trivial to add, e.g. because it requires changes to some of the core classes of LAMMPS, developers are encouraged to submit an issue about their planned implementation this way in order to receive feedback from the LAMMPS core developers about their approach and possible 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 may e-mail the core developers.