Recent first-principles studies have shown that $\mathrm{Li}{\mathrm{B}}_2{\mathrm{C}}_2$ can reach a higher superconducting transition temperature $T_{\mathrm{c}}$ than its parent material $\mathrm{Mg}{\mathrm{B}}_2$, yet its structural instability hinders experimental realization. Here, through a systematic screening of 594 ${XM}_2{\mathrm{B}}_6{\mathrm{C}}_6$ candidate systems with different metal elements $X$ and $M$, we propose a design strategy based on isovalent metal substitution to improve structural stability while maintaining excellent superconducting performance. Using a rapid screening approach based on the zone-center electron-phonon coupling (EPC) descriptor, we screen all possible ${XM}_2{\mathrm{B}}_6{\mathrm{C}}_6$ variants derived from the $\mathrm{Li}{\mathrm{B}}_2{\mathrm{C}}_2$ network. We demonstrate that isovalent $n{s}^{1}X-M$ metal pairs, such as K-Li, Rb-Li, and Ag-Na, synergistically optimize the interlayer spacing and metal-boron/carbon hybridization to retain strong electron-phonon coupling while hardening low-frequency phonon modes to enhance lattice stability. Representative quaternary systems such as ${\mathrm{KLi}}_2{\mathrm{B}}_6{\mathrm{C}}_6, {\mathrm{RbLi}}_2{\mathrm{B}}_6{\mathrm{C}}_6$, and ${\mathrm{AgNa}}_2{\mathrm{B}}_6{\mathrm{C}}_6$ are predicted to possess $T_{\mathrm{c}}$ values significantly exceeding that of $\mathrm{Mg}{\mathrm{B}}_2$ within the same computational framework, together with enhanced stability. In contrast, aliovalent combinations (e.g., Tc-Ca) can dramatically enhance $T_{\mathrm{c}}$, but at the expense of stability. These results establish isovalent cosubstitution as a stabilization-enabled route for realizing and optimizing $\mathrm{Mg}{\mathrm{B}}_2$-like superconductivity in borocarbides by improving stability while preserving the well-known B/C σ-band-driven EPC channel.

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• Electron-phonon coupling
• Electronic structure
• Superconductors
• First-principles calculations