Offer themes as well as do "Theme Challenges" to the community.
"Best new theme wins a Premium account for a year" or something like that.
Also, I would like the ability to darken, strikethrough, highlight, colour, star, etc. emails in my inbox so that I remember which ones I have already read. It can be quite confusing when you have a tonne of emails and they all look the same and there is no way to know if I have read already unless I do a completely new "Read" folder. Being able to star a special email for later or having an email title lighten in the inbox after being read really helps the user.
Plus: you could do a colour tier system of urgency for emails where the user right clicks and gets to choose if it's Red (emergency), Yellow (Urgent), etc.
These are just thoughts I have as someone who has learning disabilities and respects open source technology. Giving someone like myself the means to tweak my emails how I would like i.e. colours, stars, calendar, alerts, etc. while still maintaining privacy, security, and design would mean so much.
I don't see branching out as bad as long as you keep the community in mind the entire time.
Urgh, I have so many ideas! I wish I could work with you all and see them come into frution!