Ashes of the Singularity, by Oxide Games and Stardock, the RTS that spawned from the Star Swarm demo, will be released on March 31st. Unless something sneaks in before then, this will pretty much be our first look at DirectX 12 in a full, released game, and pretty much the only one to take advantage of its ability to draw many simple objects.
Again, I'm excluding released games based on engines Unreal Engine 4, because you don't have full DirectX 12 support if your engine provider doesn't claim full DirectX 12 support. I'm pretty sure they just enabled Epic's experimental feature, rather than beat them to overhauling their hardware interfaces. I'm also ignoring Gears of War Ultimate Edition because of the state it launched in.
It seems like the only source for this news is PC Gamer. Stardock hasn't officially said what will change in this launch from the previous beta release (Beta 2). The game currently supports mixed multi-GPU on DirectX 12, and a variety of other features, which will be interesting to see in official software. Unless we get a surprise in the official announcement, it looks like Vulkan might be a “patch after launch” thing, though.