The Razer ForgeTV looks to be a strong contender that could unseat the FireTV for modding, provided the software is open and hackable. There are a couple big reasons: Gigabit Ethernet, USB 3.0, and a Snapdragon 805 processor. This makes the device much more viable as a network attached storage device, in addition to faster media playback and access generally.
In addition to this, if the ForgeTV is capable of installing apps to an external storage device, the USB 3.0 speeds should kill any issues with performance that come with the limitations of USB 2.0.
The only real hardware disadvantage to the ForgeTV is the lack of optical audio out, which can thankfully be overcome with a usb soundcard, if that is really a huge issue for some people (it is for me- I like using spotify through my receiver without turning on my TV).
The biggest concern is the implementation of Lollipop. How the software will be locked down and what restrictions this places (specifically on bluetooth devices like wiimotes that do not work past android 4.2, and any issues caused by lollipop's handling of external storage and sdcard access) is a concern as always, and I'm sure there will be numerous issues to overcome on that front.
In any case, I'm excited for the release, and will certainly be picking one up when they go on sale. Let's use this thread as a place to discuss the transition, as I'm sure a lot of people will be jumping from the FireTV to the ForgeTV if the software proves flexible enough.