It would be possible to make mechs that no kits exist of now. Obviously scratchbuilding is also a possibility (and I am doing that as well), but this would be so much simpler and faster. I now wonder if ripping the models from PC games and using those as a basis for a 3D-print would be possible...
You could probably scan in smaller models, convert the scans to a file format that can be altered in a CAD/3D-editor, scale them up and add details before printing the parts, if the ripping doesn't work.
2 weeks ago I got my first printer, and it has been working overtime since, running free models from Thingiverse, plus some designs of my own.