Capital gains would fall into gross income but I would not tax inheritance or interest earned on investments/savings. Ok back to foreign investment. You have to differentiate investments and tax shelters. I'm no tax lawyer but basically if you are trying to hide income earned from U.S. investments, sales, salary etc in offshore accounts then I'd find a way to tax it. If you have legit foreign investments that earn income but are taxed by a foreign govt then I would not attempt to double tap that income source. This can get pretty thick in the weeds and fixing all the loopholes would take time and discipline. Ultimately make the tax code simple and the most equitable way to create revenue for the nation then adjust to catch the tax cheats.