**7. Outlook and future prospects**

Around the world, a neglected hope exists to reduce energy costs using superconductivity in power transmissions. In the nearby future of transportation, Mag-1 eV trains use superconductivity for eliminating friction to poise train cars above the rail. Who knows? Maybe one-day smartphones with long-lasting battery timing up to months or more would be manufactured based on superconductivity electronics. High-Tc era exposed remarkable production with a new superconductor family (Tc ≥ 23 K) discovered every few years in its first 25 years. Prediction of future discoveries is difficult as each class is chemically distinct from the others but some indicators are quite obvious. Future HTSCs are incredible having considerable (≥50%) nonmetal content based on known materials with high-Tc. 'Metalnonmetal' group associates very high Tc's having nonmetal contents of 40–60% with simple ionic bond considerations. Since 2008, none of any element in previous high-Tc families had featured except Fe. Spin–Spin fluctuations are necessary for the superconducting mechanism of cuprates and iron arsenide materials with highest-Tc, where transition metals are required in heavy-fermion (intermetallic) superconductor PuCoGa (Tc = 18 K) with f-block magnetism [29, 108]. In near future, such materials doped with nonmetals would lead towards tremendous discoveries. Other electronic instabilities may arise on basis of non-magnetic mechanisms, as suppression of charge disproportionation for the bismuthate superconductors. In metal-nonmetal families, chemical doping is prescribed to put an end to spin/charge-ordered ground state inducing superconductivity. This may happen due to incidental band overlap (as in YBa2Cu3O7) but sometimes by nonaliovalent substitutions of non-stoichiometry. However, disorders in metal-nonmetal's networks lead to subduing superconductivity. To obtain high-Tc's, chemical tuning of additional parts of the network (charge reservoir) manifested via difference between maximum Tc for BaBi1-xPbxO3 (13 K, essential Bi sites doping) and for Ba1-xKxBiO3 (30 K, secondary Ba sites doping). Another high-Tc materials class is portrayed by bonding among metals and nonmetals which is attributed to high content values of nonmetals (100% in case of pure organic SCs). Elements forming strong covalent bonds and networks, B and C are restricted to this group but similar nonmetals (O, N, S, P, Si) can act as dopants as well. Highest obtained Tc (41 K) so far, owing to predictions from optimal BCS (weak-coupling) and superconductivity acts like BCS in this group. For A3C60 and MgB2 (and also YPd2B2C) with proper stoichiometry, the optimal electronic structure for superconductivity is attained while Tc isn't affected by chemical doping.
