When I first started playing true RPGs just about the only game around was D&D. But I changed it around heavily, once I became used to it and had played for a couple of years.
Later on when other games started coming out, even things that weren't RPGs, like Ogre, I took ideas from them and changed D&D some.
Also books and literature, myth, religion, and science caused me to change D&D around.
Years and years later when I returned to D&D I created a system that was part D&D, part sections from other games, and partly my own creation. Some people tell me it is an entirely different game but I just think of it as a variant form of D&D, or a very different form of D&D milieu that is semi-historical and uses real world religions, and political structures, and history, and literature, and so forth. And we do things like Vad and use real world skills too.
Later on I took the same basic game construct and created a related pulp system, modern/Intel/espionage/military system, and sci-fi system for my other milieus. Which has in turn led me to rewrite my fantasy milieu into an altered setting which I guess you could say is now less D&D and more a different game. Though it started out as D&D plus other things plus my own stuff.
I love playing around with the mechanics. RPG kit bashing is almost a seperate hobby in and of itself for me. I could almost spend more time enjoying just that than actually playing (but only almost).
Indeed. Lately I have even started to experiment with mixing totally different forms like RPGs and Virtual Reality, Alternative Reality, and Parallel Reality games. I have done work in the past by creating RPG simulations for training purposes (not based on D&D but based upon the RPG type model) both for business and law enforcement (I came close to winning a simulation contract for military training purposes but lost the bid, though I did provide consultant assistance). Now I'm trying to create some simulation efforts than combine RPG, VR, AR, and PR forms.