Many people dream of a hybrid between Arduino and Raspberry Pi. As did the folks from Texas Instruments, so they created the Beaglebone Black. It combines the best of the 2 worlds: From Raspberry Pi: built-in ethernet, fast Arm processor, HDMI, runs on linux and from Arduino: two 46-pin headers that are compatible with over 30 plug-in boards called capes. Consider these like Arduino shields.
This time the price is right too: $45.
I don’t know if this will pose a threat to Arduino, but I do know that competition is good.