  Inspired by a recently proposed superconducting mechanism for a new cuprate
superconductor Ba$_2$CuO$_{3+\delta}$, we theoretically design an
unconventional nickelate superconductor with $d^8$ electron configuration. Our
strategy is to enlarge the on-site energy difference between $3d_{x^2-y^2}$ and
other $3d$ orbitals by adopting halogens or hydrogen as out-of-plane anions, so
that the $3d$ bands other than $d_{x^-y^2}$ lie just below the Fermi level for
the $d^8$ configuration, acting as incipient bands that enhance
superconductivity. We also discuss a possible relevance of the present proposal
to the recently discovered superconductor (Nd,Sr)NiO$_2$.

Superconductivity;Strongly Correlated Electrons