As as been said in several places, including here. The single weakness for many with the Fuji X system is that there is no really useful ETTL flash system supporting it. Fuji has a speedlite but no ability to create a multi-light scheme. Something that is so easy to do with the Canon 600EX-RT system. If someone would implement Fuji ETTL into their alternative system there seems to be significant pent up demand.
The problem is probably that it requires some reasonably complex protocol conversion techniques.
A) spec the protocol from the camera to a speedlite for each vendor.
B) spec the protocol from the speedlite to the camera for each vendor
C) Build a capabilities profile that covers all the selected vendors.
D) spec and write a command protocol that covers all the capabilities that should be in the "open" system.
E) Spec and write all the conversion APIs between each vendor protocol and the command protocol for both ends of the communication.
Depending on the complexity of the proprietary ETTL communications protocols that could be 1 or 2 engineer years. For Nikon, Canon and Fuji.
Cactus should have some inkling about the resource requirement based on what they've accomplished with the V6. Yongnou should have some idea based on the work they've done with the YN-E3.