I am always fascinated by new productivity utilities and ideas, and I'd like to create another one for this year's NANY 2015.
I was actually thinking of making a kind of lean and mean utility Digital Index Card Notes system, that would compliment the index-card system I have been using to keep track of todo items and ideas for many years now.
You can read about my real-world non-digital index card system
here, and a brief old discussion of some ideas for a digital version
here.
My current best idea is not to try to replicate visually the idea of physical index cards that you can shuffle around on screen, but rather to provide a database-driven system for creating "virtual" cards (i.e. small entries with title and text), with some keywords/group identification, with good fast search/sort/filtering, and then provide functionality for PRINTING such cards on
index-card stock.
So it wouldn't be a replacement for having a physical collection of index cards, but a supplemental tool to help you create, print, and search them.
Alternatively, one could imagine the utility as a more general purpose tool for maintaining a lean and mean database of entries that you want to print out on card stock (of whatever size); so think flash cards, etc.
Just my current idea, can't guarantee it will be the one i finally end up with.
Another related alternative would be not to print the entries on index card stock, but perhaps to assist in printing a daily TODO list from selected entries, or something similar.