Q:

Interesting projects/problems/challenges to work on? 

Being a "true" developer I love working on projects in my spare time.  This is not necessarily to reach a particular end but just so that I can keep my skills sharp, work with the latest patterns and ideas, and see if the latest thinking on things holds up for me. 
I don't like just doing sample bits of code, but rather try and wrap things up in a bit of a project, but I have run out of project ideas. 
Last night I sat down and came up with the following list of things I would like to cover: 

Multiple interfaces ASP.Net MVC, WPF, Silverlight
Service layer WCF
Make use of Neural Networks and Natural Language Processing
IoC, DI, etc, etc
Database, L2E / NHibernate 
SOA, multiple data sources
Uses prediction models 

What would be a good, fun demo project to cover one or more from items from this list? 
What do you guys do when you want to do something like this?  Is there anywhere that has a list of projects that people have thought of but just haven't had time to do? 

A:

Trying to build a chatbot covers quite much of your requirements.  You would need NLP and AI, some DB to store data and patterns, a service over which the bot can be queried, as well as multiple interfaces. 
Tip: make the language which your chatbot speaks domain-specific.  Let's say your bot will be able to understand and try to answer questions related to weather.  It will still be hard but at least not impossible to have some decent results.  NLP & AI for the language, and multiple data sources for pulling in weather forecast data.  You can also do your own prediction trickery on the data. 
If everything goes well you may have an impressing demo service, which you may be able to develop into something you can sell (well, with good luck.. but who knows? :)