One thing I like about many other e-mail clients, including web-based Gmail, is third-party add-ons that boost my capabilities.
From what I can tell, there aren’t that many “add-ons” that really are useful to me in eM Client. Having the ability to develop extensions would be really nice.
Such extensions could be custom widgets, mail enhancements, and so forth.
Obviously you’d want it to be fairly locked down to prevent abuse. You could always implement some sort of trusted developer system that isn’t too hard to get approved for (I’m an independent developer, so I can’t afford the hassles that many have).