The various cruiser, destroyer, and battleship designations were useful back in the early 20the century, but basically became largely useless after WW2, and certainly is pretty much entirely useless in the 21st century. There are no more battleships, and the distinction between cruisers and destroyers are neither useful nor distinguishable. Destroyers are growing as big as cruisers of old, and they can all "cruise" all the oceans (what did disappear was the idea of a nuclear-powered cruiser). There is likewise no huge difference between destroyers versus frigates or even corvettes. All that separate them is capability (number and types of weapons and sensors) and tonnage. Perhaps from now on we could use tonnage as a rough division amongst surface combatants (outside of aircraft carriers and ambitious assault ships; logistical ships aren't combatants)--say under 1000 tons for LIGHT combatants, 1000-5000 tons for MEDIUM, 5000-9999 for HEAVY, and over 10000 tons for SUPER (or maybe FAT :)). Submarines are much easier to classify by function/mission: attack-hunter subs (SSN) versus strategic ballistic missile subs (SSBN). There are cross-overs, but they're not the norm (some older Ohio-class SSBNs have been converted to carrying cruise missiles, up to some 150+ but usually fewer than that, thus serving as attack subs). Increasingly attack subs are carrying more cruise missiles and in the future perhaps even hypersonics. What will be incredibly confusing and widely diverse in the future (and even now) are the drone ships, both surface and submersibles. There will be MANY dozens of different types coming out very often in the future. The same, or actually even wilder situation, goes for airborne drones, which have had a longer history and wider use. The new trend isn't just driven by the versatility and growing capabilities of drones, but also their relatively low development and production costs compared to conventional manned ships and aircraft (at perhaps 1/100 to 1/100000). Compared to the billions or tens of billions in developing a new fighter/bomber or warship from scratch, cheap drone may take just a few million dollars to develop, and even more sophisticated ones probably stay under the $1billion threshold (with some exceptions).