What would make a modern battlecruiser?

Will there be a new class of battlecruiser, and how would it differ from a cruiser?

Only if navies again field a clear cruiser hierarchy; otherwise the name has little analytical value beyond a very large surface combatant.

The answer treats battlecruiser as a relative category rather than a displacement threshold that works in isolation. Kirov could be described that way because the Soviet Navy already operated a broad hierarchy of smaller cruisers beneath her. Many current fleets build few or no ships officially called cruisers, removing the comparison needed to define a battlecruiser above them. China can label very large surface combatants as destroyers, further demonstrating that national classification practice matters. A modern battlecruiser category would become useful only if a stable cruiser category first returned.