Here's a fun fact: back in the 1970s, US Navy admirals were quivering in their black shoes over a perceived "cruiser gap" between Us and the Soviets. As you probably put together by yourself, the Soviets simply had more cruisers than we did. However, this didn't take into account the USN's classification of a "frigate" - ships that were smaller than what were considered "cruisers" at the time but often only marginally less well armed and in a few cases (most notably the California and Virginia classes) even nuclear powered (the Virginias in particular were actually among the largest non-aviation combat vessels commissioned by the USN in the postwar period, and were the most powerful vessels on the registry at the time of their commissioning). This realization gave these admirals an extremely effective yet simple solution - reclassify these "frigates" into "cruisers." Boom, cruiser gap eliminated!
Today, we've got plenty of cruisers in the form of the numerous CG-47 Ticonderoga class - itself originally classed as destroyers and even built using a destroyer hull borrowed from the Spruance class of (for the 70s) cutting-edge, revolutionary sub-hunting vessels (it's been rumored that the designation was changed to "cruiser" to avoid the "cruiser gap" rearing its ugly head again). Furthermore, in the "Peace Dividend"-era, the War on Terror and even with China emerging as a strategic competitor, we're no longer facing a potential enemy where a "cruiser gap" would even be an issue. The People's Liberation Army Navy simply doesn't have any cruisers to speak of (not yet, anyway) and the Russians haven't built any true cruisers since the first Ticos left the slipway. At the same time, however, a new and very interesting problem is starting to emerge.
The newest Tico was launched in the mid 90s (and, incidentally, is potentially earmarked for an early retirement thanks to a very public and boneheaded move by its last captain) while DDG-51 Arleigh Burke class destroyers continue to be built with the Flight IIIs being at an advanced stage on the drawing boards. This means the Flight IIAs and upcoming Flight IIIs will actually be more capable and better able to defend carrier fleets and strategic assets than the larger yet more antiquated cruisers.
The Ticos were slated to be replaced by a program called CG(X) - Cruiser, Guided-Missile, eXperimental/neXt-gen - which, like its predecessor, was to be designed on a destroyer hull shared with the DDG-1000 Zumwalt class. Unlike the Zumwalt the CG(X) was envisioned strictly for the anti-air/carrier screening/anti-ballistic role, and would've eschewed the highly complex and heavy Advanced Gun System (AGS) for more missile space. Nuclear powerplants were floated around, mainly to power both the cruiser's SPY-3 radar and laser weapons, but mounting complexity for both itself and the base Zumwalt design resulted in funding cuts and the birth of the DDG-51 Flight III. Ultimately, both the Zumwalt (at least in its initial envisioned form, also with SPY-3 and SM-6 ABM capability) and DDG-51 Flight III would've made the dedicated anti-air/carrier screening/ABM mission of the CG(X) redundant - leading to a valid question, what was its true purpose to begin with? It's possible that the whole point of CG(X) would've allowed for the USN to continue fielding cruisers, even though the capability gap between destroyers and cruisers (and even frigates) is rapidly shrinking.
The Royal Navy's analogue to the DDG-51 is the Type 45, equipped with the SAMPSON and S1850M/SMART-L radars which combined have a relatively narrow capability gap compared to the DDG-51 Flight IIA's SPY-1 radar. Compared to the DDG-51, the most limiting factor of the Type 45 is its "single-ended" configuration with only one vertical launch bay for missiles. The Type 45 is broadly representative of a new trend with upper-end yet budget-constrained navies: smaller ships that try to max out what it can do on a sub-10,000 ton hull while still retaining critical capabilities, namely anti-ballistic missile and "exo-atmospheric" interception. Raytheon has been shopping around the SM-3 missile's exo-atomspheric capability to European navies equipped with similar ships, including the Royal Navy for use onboard the Type 45s. Most of these ships are classified as "frigates" for "political reasons" (i.e., to make them more politically attractive to parliaments and public voting bodies that keep voting in favor of shrinking budgets and reduced aggressive stances in foreign policy) even though they displace as much as some WWII "light cruisers" (yet another naval classification that has its roots in politics). The Royal Navy actually considers the Type 45s to be proper destroyers.
On the other end of the spectrum, the Republic of Korea and Japan have been building "Arleigh Burke clones" such as the RoKN's King Sejong the Great class. Though almost visually identical to its USN counterpart, the vessel is a stretched and enlarged version pushing well over 10,000 tons - greater than the displacement of the Ticos (and into the same displacement territory as nearly every cruiser to set sail save for the battlecruisers of WWII and the monstrosities that are Russia's Kirovs). If going strictly by comparing tonnage to contemporary ships, it would rate a cruiser in the USN. If going strictly by actual capability it would still rate a cruiser - it carries more missile tubes than a USN destroyer and just a bit shy of what a Tico can do, has a more advanced radar than most Ticos and it's highly doubtful that having one less measly five inch rifle in the "missile age" will cheat it from a cruiser rating (it still has a larger caliber gun than Kirov herself). Zumwalt herself will have a larger displacement than nearly every cruiser the USN ever commissioned, up to and including the nuclear Virginias and even the Des Moines, the ultimate USN "all-gun" cruiser. Only the Alaska "pocket battleships" boast a larger displacement other than including true dreadnought battleships (some of the earliest of which Zumwalt will still out-displace).
It should be evident, then, that in an age where capability is becoming more and more independent of hull size - and as "destroyers" start to out-size cruisers - the various classic hull designations between "frigates," "destroyers" and "cruisers" start to become meaningless. Furthermore, these classifications actually risk running afoul of political inconvenience or simply falling victim to various "brain bugs" - the USN will have cruisers, dammit, even if it means gussying up a destroyer hull. This leads to meaningless nonsense like the 1975 Reclassification Act and "frigates" capable of bringing down orbiting satellites.
Therefore, it's time for a deep rethink and reconceptualization of the US Navy's classification system.
Interestingly enough, one of the USN's most controversial procurement programs (besides Zumwalt) points to a very logical path. The LCS - Littoral Combat Ship - tells you exactly what it is regardless whether you're a Navy layman or an average taxpaying citizen. This type of classification describes the ship based on mission - a ship designed to do combat in the littoral regions - rather than by an arbitrary rating based on tonnage and obsolete concepts of firepower (once upon a time, the classifications "destroyer" - as in torpedo boat destroyer - and "cruiser" - a ship designed to cruise independently on long-range missions - did the exact same thing, but these classifications have long since carried meaning more relevant to tonnage ratings). Following this lead, every major surface combatant of the USN can be reclassified in a similar manner. Actually, speaking of which....
Major Surface Combatant (MSC)
Like the LCS classification, the MSC classification (more or less) tells you exactly what the mission of the vessel is - to conduct major surface operations, whether as part of a surface action group or more likely as part of a carrier screen. This would include all ships traditionally considered destroyers and cruisers, as well as most European high-end frigates.
Patrol Escort Ship (PES) or Multi-Mission Combatant (MMC)
Of course, you still need specialized ships for "other" than "major surface actions" such as convoy escort. This mission is currently fulfilled by what the USN calls "frigates" like the FFG-7 Oliver Hazard Perry class. In the future, it could be fulfilled by "Patrol/Escort Ships" like the "Patrol Frigate" shipbuilder Huntington Ingalls is pitching based on the US Coast Guard's National Security Cutter. Alternatively, a catchall "Multi-Mission Combatant" classification can cover miscellaneous roles over a very broad and flexible range - a term coined by shipbuilder Austal as they pitch their "LCS-on-steroids" concept based on the tri-hulled LCS-2 Independence class. In either case, such a vessel would be affordable but defensible against a wide range of threats and be more economical to operate than larger vessels used for carrier screening (often equipped with the same type and number of gas-guzzling turbine engines as a 747), making them appropriate for patrol missions and convoy escort. Their smaller size would also make them more flexible in the littoral regions, perhaps lending support to LCS-type ships.
At the height of the Cold War, the USN had a wide diversity of ship types and classes, from the frigate/destroyers of the Farragut class to the frigates/cruisers of the Belknap and Leahy classes and California and Virginia nuclear ships up through the "true" cruisers, the Albany WWII-era construction ships which went through missile conversions, the Long Beach nuclear cruiser and the various destroyers of the Forrest Sherman, Charles F. Adams and Spruance classes. Today, the USN is nearly uniform, with only two broad classes of major surface vessels - the Ticonderoga cruisers and the Arleigh Burke destroyers which now exceed the capability of the so-called "cruisers." With only three examples of the Zumwalt set to join the fleet - and with the Ticos being pushed out in favor of the "Burke-spam" - the fleet will only continue to grow more uniform. Technological developments have meant more capability on a single hull form, and it's time for the USN hull classification system to reflect that uniformity and technology.