  We predict a new class of three-dimensional topological insulators (TIs) in
which the spin-orbit coupling (SOC) can more effectively generate a large band
gap at $\Gamma$ point. The band gap of conventional TI such as Bi$_2$Se$_3$ is
mainly limited by two factors, the strength of SOC and, from electronic
structure perspective, the band gap when SOC is absent. While the former is an
atomic property, we find that the latter can be minimized in a generic
rock-salt lattice model in which a stable crossing of bands {\it at} the Fermi
level along with band character inversion occurs for a range of parameters in
the absence of SOC. Thus, large-gap TI's or TI's comprised of lighter elements
can be expected. In fact, we find by performing first-principle calculations
that the model applies to a class of double perovskites A$_2$BiXO$_6$ (A = Ca,
Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, more
detailed calculations considering realistic surface structure indicate that the
Dirac cones are robust against the presence of dangling bond at the boundary
with a specific termination.
