I'm actually making a couple of models of ships like this, and have some concepts for a few more. Here are some ideas:
Gunboats: Basically a weapons battery (beams or missiles/torpedoes) with an engine attached. Small (size of a heavy cruiser - no more than 500m or so), lightly armoured, but massive firepower. Designed for delivering a decisive blow in a tactical battle situation (ie crippling/destroying a key enemy vessel such as a corvette or carrier ship), or for hit-and-run attacks against larger strategic targets (destroyers, installations, fleet marshalling points). They lack the durability of conventional "ships of the line", but their firepower gives a commander a powerful shock weapon with which to break up an enemy formation or destroy strongpoints such as capital ships and installations. Practically useless in defensive situations, however.
Strike cruisers/corvettes: Medium warships, between cruiser and corvette size (400-1000m). They have reasonable armour, a high top speed, and carry specialist armaments. This will typically be a heavy weapons battery (beams, plasma burst turrets or missiles/torpedoes), a hangar carrying fighters, bombers and assault boats (basically boarding craft) backed up by heavy flak and AA turrets, or a combination of the two. However, individual ships may be retro-fitted with different equipment for highly specialised tasks such as intelligence-gathering, "electronic" warfare (ie jamming, tracking and misdirecting targets), or even transport of highly valuable cargo and/or persons. These ships are mainly employed by covert units such as the SOC, but a few are used by elite navy units for strategic operations during times of conflict. They are highly mobile vessels, and their equipment allows them to excel against their targets. However, the specialised nature of their armaments means that they can struggle if they get into a situation they are not equipped for.
Escort carrier: Fairly small vessels, with limited direct offensive capability, but with a fighterbay. They provide a platform from which to launch and recover fighters and bombers. They have a small capacity, but a high speed, meaning they can keep up with the fastest transports and capital ships. Typically they will be no larger than an Aeolus-class cruiser, and will mount no heavy ship-to-ship weapons, but will carry up to three wings of fighters or light bombers. They are principally used to support high-risk convoys and remote operations, where a capital ship cannot be spared, but they can also be used to mount long-range surgical strikes against targets deep behind enemy lines. Their small size and high speed means that they can evade detection where a destroyer or tactical carrier would be tracked easily. However, they are very vulnerable to heavy weapons such as beams and torpedoes, and will not survive a determined attack without cruiser support.
Artillery cruiser: Heavy cruisers, these ships mount long-range heavy weapons, including heavy beams, bombardment cannons (railguns) and missiles. They are used to hit targets at long range, usually installations and planetary targets, beyond the reach of static defences. They are poorly equipped for ship-to-ship combat, their main weapons typically having lower refire rates than those on other ships. They fare even worse against fighters and bombers, having few defensive turrets and little armour.
Monitor: A type of cruiser, these ships move rather slowly, and cannot maneuvre to any great degree. However, they have thick armour, and mount a very heavy weapons battery for their size. They will usually carry at least two beams, a bank of several plasma burst cannons, torpedoes or heavy swarming missiles in addition to heavy AA and anti-warhead turrets. They are basically mobile gun platforms, designed to defend key locations or blockade jump nodes. Their firepower is formidable, and most ships will take heavy damage in a direct assault. However, their sluggish movement means they can be outflanked by a clever opponent, and their heavy weapons mean that they have no reserve power for subspace travel. The Leviathan could be described as an early example of a monitor-type vessel.
