Principle langauges have just emerged. The future will show if they will be forgotten or explored further. In the latter case there are plenty of possibilities for research:

• Examining single principles: The vast majority of principles hasn't been subject to empirical studies, yet
• Examining the OOD Principle Language: Further experimentation may be done, e.g. wrt. certain domains (information systems, embedded systems, etc.) and different experience levels of designers (students, practitioners, highly experienced designers, etc.). Also its value for teaching could be examined.
• Finding further principles: Certainly there are more principles than are described here.
  • Literature research: Certain principles may already be described in books, papers, blog articles, wiki pages, etc.
  • Ask practitioners: Another source may be asking practitioners for their unwritten principles.
• Further Principle Languages
  • Other forms of software design: user interface design, framework design, database schema design, communication protocol design, or mobile application design, …
  • Tailoring Principle languages for specific domains: for the design of control systems, enterprise applications, games, multimedia systems, aerospace applications, software for the the automotive industry, …
  • Principle languages for certain non-functional requirements: performance, reliability, security, …
  • Principle languages for other levels of abstraction: architecture, requirements analysis, algorithm design, coding, …
  • Principle languages for other programming paradigms: procedural programming (C, Pascal, …), functional programming (LISP, Haskell, …), fourth generation programming languages (Progress, ABAP, …), scripting languages (bash, ruby, python, PHP, …)
• Metrics: Maybe the principle language approach can be combined with metrics. One could envision a tool which rates designs or code based on a special-purpose principle language where each principle has a corresponding metric. It is still an open question if this would be helpful or not.
• …