The ability to write by hand in the journal would be a fantastic feature for me. As you may know, "Day One" offers this functionality, but it's limited to the iPad. After extensive research - I actually spent over 5 hours looking into this - I couldn't find a journaling app that provides this feature for both tablets and PCs. Introducing this feature could set you apart significantly from other software.
To start, simply being able to write, much like with a pen, without additional features such as various colors, would be more than sufficient.