Concerning Fn-Layer: I'd like to be able to access the media functions with one hand, e.g. with Fn on the right, as it is now, and all the media functions (including forwards/backwards and play) on the right cluster, where PgUp adn PgDn, etc. is. That way you could press Fn with your thumb and the media keys with the other fingers.
Concerning Layout: I like x88-DE__F1-above-2 aka 88b_c137/Jesterfox the most, but ISO is a must for me.
Backlight would be very nice, but is not absolutely necessary.