The designer of Radlands has a website with lots of articles about various aspects of board game design. We’ve pulled out some of the best on design theory:

https://daniel.games/the-depth-complexity-ratio.htm

https://daniel.games/relatability.htm

https://daniel.games/big-moves.htm

https://daniel.games/calculation-vs-intuition.htm

https://daniel.games/ramps.htm

https://daniel.games/interlinking.htm

https://daniel.games/player-interaction.htm

https://daniel.games/work.htm

https://daniel.games/tension.htm

https://daniel.games/feelbad-situations.htm

https://daniel.games/awesome-things.htm

https://daniel.games/types-of-fun.htm

https://daniel.games/accumulation.htm

https://daniel.games/consequential-actions.htm

https://daniel.games/reducing-complexity.htm

https://daniel.games/randomness.htm

https://daniel.games/resources.htm

https://daniel.games/access.htm

https://daniel.games/spatial-systems.htm

https://daniel.games/making-sure-the-game-ends.htm

https://daniel.games/scaling.htm

https://daniel.games/aesthetic-patterns-in-design.htm