I like Imogen, but I don't think it needs a nickname. English isn't my first language which might be why, but I confuse Imogene with Imagine, and thus I prefer Imogen.
I like it without a nickname too; I was just trying to think (far) into the future, as my family tend to be nicknamers. Imogen and Imogene both remind me of the word imagine-I think that's why I like it so much.