Support for 5 extruders.
I just purchased a diamond 3-in-1 hotend. The problem lies in that Repetier Host only can control two of the extrusion motors. There are already hotends with 4 extruders in 4 heat blocks (which would need the ability to control both the four motors and the temperature in four heat blocks. My understanding is that one company is already designing a 5-in-one hotend for Cyan, Magenta, Yellow, Black and White mixing. So if you are making room for three extruders, you may as well make room for 5 for future expansion.