The recent discovery of "polar metals" with ferroelectriclike displacements offers the promise of designing ferroelectrics with tunable energy gaps by inducing controlled metal-insulator transitions. Here we employ first-principles calculations to design a metallic polar superlattice from nonpolar metal components and show that controlled intermixing can lead to a true insulating ferroelectric with a tunable band gap. We consider a 2/2 superlattice made of two centrosymmetric metallic oxides, La_{0.75}Sr_{0.25}MnO_{3} and LaNiO_{3}, and show that ferroelectriclike displacements are induced. The ferroelectriclike distortion is found to be strongly dependent on the carrier concentration (Sr content). Further, we show that a metal-to-insulator (MI) transition is feasible in this system via disproportionation of the Ni sites. Such a disproportionation and, hence, a MI transition can be driven by intermixing of transition metal ions between Mn and Ni layers. As a result, the energy gap of the resulting ferroelectric can be tuned by varying the degree of intermixing in the experimental fabrication method.