Hello Demetris, will soon be contributing financially your projects!

May be MAVlink can be used to replace Xbee protocol.

It's just an idea, I'm not an expert on the subject, I'm just trying to find a cheaper replacement for the xbee hardware and the hardware below is OSHW and suport 433MHz(long range), can be a good option.

www.hoperf.com/upload/rf_app/HM-TRP.pdf

I think it's better to stick with the xbee standard, because, the xbee modules aside, you can change module in the future and upgrade as new modules are released, there are many different manufacturers and there will be even more now that this "internet of things" movement is starting to gain momentum.

Also, mavlink is just a protocol, I think you can use it over xbee if you want. You could modify the OS-RC code to only communicate via mavlink so your drone can speak mavlink with the OS-RC receiver modules directly. This is the power of having open software

Like mentioned earlier Mavlink is just a protocol. It could be supported by the FPVC Advanced unit later on.

I haven't found a better alternative for Xbee so far and everything works fine with it si i see no reason in replacing it for now.