I'm definitely creating my own system. That way I can ensure the best fit to what I want to do. It is also nice to have that freedom, rather than being obliged to use an established system associated with the intended recipient of the adventure.
Before the Gamebook zine started up, I was looking at writing a adventure for the Fantasy Fanzine but I really was not fussed about having to use a system like the Fighting Fantasy one and was wondering how far I could get away with deviating from the standard rules.