Sure, there are tons of fundamental concepts. Even in what you mentioned - what are the types of taxes that exist and whom do they impact? How are taxes levied,

Sure, there are tons of fundamental concepts. Even in what you mentioned - what are the types of taxes that exist and whom do they impact? How are taxes levied, how is tax burden calculated, and within each society, how is that changed? What's regressive, proportional, or progressive taxation - for bonus points, what is the historical impact of these changes? Bigger picture for underlying concepts - what is money? What are the historical monetary systems that have been developed? What is debt? How does fiat currency work and how does this system work globally? A basic discussion of inflation/deflation, monetary theory and practice. How does fractional-reserve banking work (relation to liquidity, money supply, and velocity of money). I'd add that there are a number of non-intuitive fundamental concepts for investing: the time value of money, compound rates (bonus: calculating differences in growth from tax-advantaged accounts), what is rebalancing and why does it work (related, how does correlation work in improving yield), or even understanding the basics of "capitalism" (specifically the theoretical and practical implications of rent-seeking and accumulation of capital).