Solution for simple multi-Arduino communication network (or alternative?)
Thanks all for your answers. 

I did some more research with the new info you provided and found my perfect solution: TCA9548A. That's a 1-to-8 port i2c multiplexer that will allow the master MEGA2560 to control up to 8 independent i2c buses, where I can connect my displays. The chip is dirt cheap here - 2 euro only, and it is available in the store.

Thanks again guys!