Either the most cursed or the most blessed idea I have ever had:
Strongly Typed Typesetting System
@robryk I thought that just like in any programming language, a couple base types and you can define more wherever. Preferably dependent types, these are always cool.
Disclaimer: I have no idea why any of this would be useful, especially dependent types.
@timorl With types defined at which level? E.g. in each document style?