As a plasma table is something I keep thinking about (if I do build one, it will have to be designed to live outside!), here's some of my thoughts.

For ease of access/manoeuvring sheets, consider swapping the gantry 90deg to the long axis. Depending on use/position, it means it'll be less likely to be in your way when loading/unloading the table.
Allow for adding a water table.
You'll want more legs, with some good triangulation for additional support.
I'd personally favour using servos, especially at that size.
I'd design the linear rails so they're more protected from the plasma dust settling on them. My idea would be to have the rails sit within their own open sided channel, which only has a slot for the gantry. The slot can then be further sealed using some brush strip, so any dust can't directly fall on the rails.