  Attempts are made to unify gravity with the other three fundamental forces of
nature. As suggested by higher dimensional models, this unification may require
space and time variation of some dimensionless fundamental constants. In this
scenario, probing temporal variation of the electromagnetic fine structure
constant ($\alpha= \frac{e^2} {\hbar c}$) in low energy regimes at the
cosmological time scale is of immense interest. Atomic clocks are ideal
candidates for probing $\alpha$ variation because their transition frequencies
are measured to ultra-high precision accuracy. Since atomic transition
frequencies are functions of $\alpha$, measurements of clock frequencies at
different temporal and spatial locations can yield signatures to ascertain such
conjecture. Electrons in highly charged ions (HCIs) experience unusually
enhanced relativistic effects. Hence level-crossings can be observed often in
these ions compared to their isoelectronic neutral or singly charged atomic
systems. Such a process features by their more significant relativistic
sensitive coefficients ($q$) of atomic transitions. For unambiguous detection
of subtle changes in the transition frequencies due to $\alpha$ variation, it
would be judicious to contemplate transitions for which $q$ values are
enormous. HCIs are considered one of the most suitable candidates for making
atomic clocks as they are the least sensitive to external electromagnetic
fields owing to their exceptionally contracted orbitals. The first HCI clock
has been realized, but its accuracy is much less than the counter optical
clocks based on neutral atoms and singly charged ions. The realization of HCI
clocks can add an extra dimension to investigating fundamental physics. In this
work, we survey HCIs suitable for clock candidates on the grounds of general
features, including their potential to probe temporal variation of $\alpha$.
