What are these universal principles in your federal Constitution?
These are: a) a vertical division of powers (Kompetenz Katalog), b) a horizontal division of powers (trias politica), c) checks and balances (equilibrium between the branches when they need each other), d) dualism (parliament and president have their own democratic mandate), e) bill of rights, f) the federation is of the citizens), g) mutual rights and duties between the states and between the federal body, states and citizens. That is all there is. This has proven to be sufficient to unite 50 states in America. Who dares to say that this would not be applicable to Europe is talking nonsense. The United States of America and other large Federations have proven the contrary, for many decades.