Since both engines are now open source(!), there's a possibility of a confluence between the two projects... but there's no immediate plans for any Amnesia to Thief style mods. It looks like the mission editor for Amnesia could handle many of the elements but the AI would need SIGNIFICANT new changes unless you only want undead or haunt missions...
The Dark Mod v1.07 is out!
* International language localization support for missions.
* Translated HUD, GUI and Menus.
* Vine Arrows !!!!
* New combat animations and sounds
* Different Melee damage depending on weapon class
* Un-lockable Mines !!!
* New textures and compression of existing textures to DDS
* New models including Jack-o-lanterns, water tower, cloth doll, custom keys (with skins)
* New AI "waif" models
* Steam fix for problems with game saves and installing missions
* Stealth Score fixes
* Vastly improved AI behavior:
"It’s hard to describe all the ways our AI have become more sophisticated in 1.07, mostly thanks to grayman. AI will now properly notice and comment when suspicious doors are left open. They’ll notice rope arrows left dangling in lit areas, and will come over and look up or down the rope to see if anyone is on it. AI will notice if arrows are left sticking out of things (and will no longer call them “weapons”, or comment casually after getting shot with them). AI will do a better job of warning each other about suspicious events now too... If an AI sees the player but fails to catch him, he’ll warn all his friends that there is a thief in the area. If he sees a dead body, he’ll start warning people about a murderer. This causes the friends to start patrolling with their weapons out, ready for trouble. All the new interactions make the AI seem even more believable and challenging."