They say that in programming, implementation is everything while ideas are a dime a dozen. These ideas you can have for free.
The following are programming concepts I want to do at some indeterminate point in the future, but for some (probably bad) reason I haven't gotten around to yet.
A Browser-based Clone of the Game of Thrones Boardgame
The game relies on a bewildering array of tokens and sliders which are hard to keep track of at a gaming table. It is at its most fun when players have an opportunity to communicate covertly, which is very easy in an online environment. The game takes hours to play, and hours can be difficult to find. I think it is an excellent candidate for a "background game" that people have open in a browser tab and switch to when something happens.
A dear friend who is much better at games programming than I am has said this would be simple to implement using a game framework. Personally, I was just going to abstract territories and other graphics down to simple polygons, draw on the HTML 5 canvas, and perhaps figure out how to use something like meteor to handle the communication between clients.
Incarnum Tracker
In Dungeons and Dragons 3.5, keeping track of bonuses when playing a meldshaper who like to vary their essentia investments is a horror. This could be done with an app - most likely a web client of some sort.
Initiator Leveller
Again in Dungeons and Dragons 3.5, leveling up a martial initiator is a difficult thing to keep track of, due to the intricacies of how maneuvers are interdependently learned and "forgotten". This could be managed by providing a UI representing each level as an element on a stack.
A Graph Theory Textbook
Reynir Axelsson wrote an interesting little textbook on graph theory a decade or three ago. Unfortunately, the original digitalized copies of the work are lost - what we have is a scanned copy of a printout, which has obvious limitations.
One day (if I obtain the author's permission, I have not even discussed this with him directly), I think it would be very rewarding to get this book on to a modern, computer-readable format. Still have not decided whether this would be better as a LaTeX document (which would be prettier) or a website (which would be more accessible).
A full read API for the State Alcohol Store
My beer search is already scraping a limited subset of the state alcohol store's dataset. This could be extended to a full API with relative ease. I am leaning towards using Django + the Django REST Framework for it, but this could be a great opportunity to get more familiar with something like sails.js.
DNA-brainfuck
Brainfuck is a fun language, but suffers from being overly readable. An improved version would replace brainfuck's symbol system with combinations of the four bases present in DNA molecules (ACGT).
The project DNA# exists, but I have not seen the source code.
Retro Games
Writing clones of older games is fun. Supaplex is one that's dear to me and I think I could handle, and I have not seen anything but a Flash remake of it.
Cowsay as a Service
The important Cowsay program clearly needs to be implemented as a microservice.