Farlands are Void

Beyond the barrier is Void. That's it. Nothing else. (Please insert appropriately eery, outer-space inspired soundtrack and ambience). 

The biome before the Void is simply a transitional biome. I recommend one of the following: 

'Glitched' - natural blocks we are accustomed to are speckled through a 'void block.' The 'void block' is indistinguishable from the void to the eye. However, you can walk on it. (Think "Dimensional Doors" mod or "What if Minecraft had a Void update" by SystemZee)

'End': Beyond the barrier is The End and, in addition to the void fade out there is an endstone fade out. Then there's something to see and the implication is that the barrier between the dimensions is thin here rather than something is eating away at reality. 

'Impact Site': A destruction of world gen that makes the world look like an enormous meteor crashed here. Loads of lava and fire and enormous pits from where the meteor. Maybe a new block-type associated with meteors. 

'World's End': (My personal favorite) Oceans that fall over the edge of the end of the world. If you sail over the edge, you are teleported back to spawn with all your items. 

TLDR; Beyond the boarder is Void and the new, Farlands-only biome exists as a transition to Void.