  We examine energy level-crossings of fine-structure (FS) levels in the
heavier highly charged ions (HCIs) with $d^6$ and $d^8$ configurations. From
the analysis, we find that some of these HCIs are tailor-made for atomic clocks
with quality factors ranging from $10^{16}$ to $10^{18}$ and fractional
uncertainties below $10^{-19}$ level. Many of them are also realized to be
highly sensitive to the observation of temporal variation of FS constant
($\alpha$) and testing violation of local Lorentz symmetry invariance (LLI). To
probe variations of $\alpha$ and LLI, we have determined their corresponding
sensitivity coefficients in the investigated HCIs. Similarly, we have estimated
orders of magnitudes of the Zeeman, Stark, black-body radiation and electric
quadrupole shifts of the clock transitions of the considered HCIs in order to
demonstrate them as the potential candidates for atomic clocks.
