I've liked the standard clip in sony covers - my 'old' (but only recently old) 505 and now a 350... the little things like the internal magnets to gently hold it closed, and so on.
I do wonder, though - how about a half-a-cover option.... one of the pleasurable things about a Sony reader is the reader's body feels good - solid, weighty without being heavy. So make it one side cover only - leave the back of the Sony uncovered with the cover closed, then when you open the cover it folds back to cover the back with one layer of leather only.
And have some cool inside leather cover colours and designs.... Paul Smith etc
Coops