What next?

So I thinking of writing a new website project in Ruby (probably Rails), but I’m stuck on an actual idea. I know what I want to play with:

Languages: Ruby on Rails, JavaScript
DB: MySQL (as Dreamhost doesn’t have PostGres) 
Test frameworks: rSpec, Selenium
CI: CruiseControl.rb

At the moment I’m thinking of porting the lovely RoundCube mail project from PHP to [...]