A modern battlecruiser would exceed an enlarged destroyer

What might a twenty-first-century battlecruiser look like?

The concept is much larger than an enlarged destroyer: a huge nuclear-powered surface combatant comparable to a bigger Kirov.

The question proposes an enlarged Type 45-style command ship as a possible twenty-first-century battlecruiser. The resulting concept is a much larger nuclear-powered surface combatant. Its shorthand is a ship in the Kirov lineage, only bigger. The speculation makes scale and propulsion the features that separate this category from an ordinary destroyer.