I too think that a color mixing printer would be interesting, but voted for CoreXY because that is the direction my own FT5 is slowly headed.
It would have been great to be able to purchase that "right out of the box".
Although a 500x500 printer could be very interesting and indeed unique, I suspect that the market for it might be limited by price and also because most people just don't actually need that large of a printer.
The vast majority of things I print on my FT5 for example are less than 100mm in any direction.
But there are those occasional things that push the bounds, which is why I wanted (and do use) 300x300x400 to start with..........
Although not listed as an option, I would also have voted for a Smoothie board and touch LCD setup (which I also have and really like) but that was not available as an option.
An MKS clone version would be OK to keep costs down.
That would be a modest cost adder over the standard RAMPS board and display and would be a great option for the FT5 kit if nothing else.
Other nice options would be a standardized cooler and a bed level sensor.