The mixed reality community sees a future where apps react to the user's surroundings in a meaningful way. To enable these complex and adaptive experiences, development tools must evolve: developers will shift from designing experiences for a known virtual world, to instead thinking in terms of rules, conditions, and fallbacks. Developers need to be able to create games and apps that can not just run in, but really interact with, the user's home, office, bus, city street, or empty field. Flexible layout and rich understanding of the user's surroundings are vital. Simultaneously, these tools must be accessible not just to hardcore procedural content programmers, but also to 3D artists, game designers, architects, advertisers, and 2D app developers new to 3D and MR.In this talk, toolsmiths from Unity, Magic Leap, and Microsoft Research discuss the challenges of enabling this MR future, and debate the approaches to overcoming them.