I think that, at some point, relative to size of part, the build plate/table will need to become stationary, as I have some "Things" that get very heavy for my FT-5 due to the addition of inserts that get sealed inside.
A three Axis Gantry would keep the weight of a part away from affecting anything.
One thing I believe would improve the machine, would be to replace the belt drive with direct drive Ball-Screws.
Yes, that will jump the cost WAY up there. probably prohibitive. However, I am considering modifying my next FolgerTech machine to include Direct Drive. But, we are getting into a need for position feedback, complicated controllers.
Multiple Hot-Ends would be really cool. A Controller Display with a Joy-Stick would be great too!