It's interesting that you mentioned modularizing the different components. I was thinking along the same line for my cnc router that I building. I plan on making the y axis its own complete subassembly or modular axis that can be moved as a complete unit. Since one of the design problems of raising the y axis high from the x axis base is the introducion of flexing to the gantry, it occurred to me to make the y-axis movable so that it is close to the x-axis when I want to do aluminum routing and high off the x-axis when I want to router a 10" columns (on a 4th axis). Making the entire y axis assembly (complete with main beam, motor, ball screw and z-axis) movable on the two side supports of the gantry makes for a more flexible and accurate machine. And it isn't all that hard to do nor does it add significant amounts of weight to the gantry.
Benny asked my if I wanted to make my machine open source and I mentioned it would be fine (I'll make my millions elsewhere When I finish my machine, I'll donate its design and my notes to the open source community. It can be improved upon and refined by the millions of cnc'ers out there. (I may even incorporate some of the improvements on the original!)