A long time ago, on a blog-hosting site far far away, there was a blog called ArcDeveloper (not to be confused with ArcDeveloper.Net, which came later and did more). That was my first blog. It went in a few different directions. I wasn’t happy with it, so I killed it. There’s nothing to link to now.
Anyway, one of the posts I did on that blog had to do with creating derived geometry classes in .Net. The basic premise is simple: create a class that inherits from one of the ArcObjects geometry classes and then extend it to fit your needs. My application at the time had to do with writing an ArcGIS client for Common Alerting Protocol (CAP) feeds.