I think I have the same sickness, but a different strand of it. As many of you know I am a Machinist and have built planes in the past (Lie Nielsen) and am working on a few of my own hand planes now (shoulder plane).
The thing is as I make boat parts, it gives me all kinds of time to think. So as I work I think about what my next plane is going to be, how I am going to make it, what I need for steel and other materials. Yes it is indeed a sickness.
I think my next plane will be a Router Plane. The last day I worked for Lie Nielsen we started production of one based on the famed Stanley Router Plane. They were easy to produce and even building Lie Nielsen Planes I had a few revision changes I would do to make them better. I think that will be my next plane I make anyway. Then maybe a scraper plane...then maybe a adjustable mouth block plane...and then...