Problem: There is only one Midline Capital in VanillaThe venerable Conquest, a very good (albeit unconventional) Midline battlecruiser is the sole Midline ship among the Capitals. I have nothing against the Conquest: I just think it's lonely! High Tech has the Paragon, Astral and Odyssey. Low-Tech has the Onslaught and Legion. You also have the Pirate Capitals in the Atlas/Prometheus Mk. II and the two logistical haulers (Atlas and Prometheus).
Solution: Add a Midline CapitalOf course, the question is: what would a new Midline Capital look like? We're not adding ships just to add ships: it needs to serve a distinct function that isn't already available. To be fair, I think most of the roles for Capitals are already handled. You have two battleships (Paragon & Onslaught), a pure carrier (Astral), a heavy battle carrier (Legion), two nimble battlecruisers that each have their own flavor (Conquest and Odyssey) and then the "chaff" capitals that the Pirates have. Both of these are paper tigers.
Suggestion: A "Command" CapitalMidline is known for two things: very balanced generalists and hyper-specialists. Look at a Centurion, Hammerhead, Falcon, or Eagle. They're all very middle-of-the-road in terms of overall firepower, speed, armor, etc. They don't have glaring weaknesses, nor do that have glaring strengths. However, within the Midline line-up are glass cannons (the Sunder and new Champion Heavy Cruiser), the Gryphon (missile boat) and Heron, which is a dedicated fast-carrier. The Conquest itself is hard to pin down in either of these two categories. It is, on the one hand, a generalist: it has good coverage everywhere, is generally fast, well-armored, and has excellent flux stats. However, its poor shield and unorthodox broadside nature kind of puts it into a specialist category. If anything, the Conquest epitomizes the Midline fleet doctrine: it is
balanced and is tactically flexible.
So, my suggestion is to build on that and have another "balanced" Capital that is a bit of a jack-of-all-trades but has a little specialty: namely in acting as a force multiplier. I'm dubbing this a "Command" Capital because not only does it bring a generally "balanced" battle profile, it
creates targets of opportunity.
The ShipA picture is worth a thousand words, I suppose. This is a rough mock-up of what I had in mind.

To explain: Yes, it looks like a Bird. I'm calling it the "Aquila" right now (I know, we have an Eagle but...) The hull profile/name is obviously malleable! Firing arcs are the solid colors in the colored circles (not the negative space!). I hope the color-coding and relative sizes tell you mount types.
The Aquila is built around 2 Large Ballistics and 1 Large Energy, both turreted and forward-facing. These 3 main batteries are flanked by 2 Medium Ballistics and 2 Medium Missiles on the "wings." In the "head" of the ship, 2 Medium Energy and 2 Small Ballistics give ample coverage facing forward. For rear-defense, 2 Small Energy in the "wings" face backwards and two Medium Energy and a Small Ballistic cover the "tail."
3 Fighter Bays complement the ship's weaponry, giving tactical flexibility and long-range punch. This rounds-out the "jack-of-all-trades" idea that Midline is known for.
Technically, the Aquila can bring more sustained firepower to bear than a similar Conquest broadside however its flux profile is nowhere near the Conquest's and the Conquest, overall, has a much greater armament. It is also conspicuously lacking a true missile component. It will also not be as nimble as the Conquest (it is the same speed but doesn't have Maneuvering Jets) so it needs to be able to trade fire with other Capitals and not feel under-gunned.
Overall, it is not bristling with weapons nor does it have a lot of Small mounts available for PD. It will be vulnerable to smaller swarming attacks, especially on the rear and sides. However, its forward firepower will be impressive and it will be able to take a hit with relatively good shields/coverage and good-not-great armor/hitpoints.
The System: Focus FireThe Aquila, as a "Command" Ship, can be a lynchpin for a successful assault. On-board analysis of an enemy ship can be relayed to all allied craft to pinpoint weaknesses in the shields and hull (not unlike a Heron's Targeting feed, but on a fleet-wide scale). As such, a targeted enemy ship will take an additional 25% damage from all allied weapons.
Focus Fire will give the Aquila the ability to "mark" a target. My current thoughts on this is an 8-second duration with a 15 second recharge, with 2 charges. Ships will have to be targeted (i.e. "R" selected) or the ability will not work. The 25% damage bonus is
after all other modifiers. Whether or not the AI can be "persuaded" to attack "marked" targets, I don't know, but an Eliminate Command + Focus Fire may be a wise use of Command Points (which you will accrue faster anyway, more on that later).
Current thoughts on stats:
Hull Stats:OP: 325
Hull: 14,000
Armor: 1250
Shield: 270 degree Front Shield
Shield Efficiency: 0.9
Shield Upkeep/sec: 300
Top Speed: 45
Flux Stats:Flux Dissipation: 900
Flux Capacity: 15,000
Logistic Stats:DP: 45
Burn: 8
Fuel/ly: 10
CR/deployment: 15
Crew: 500/700
CR Recovery Rate: 3%
Built-In Hullmods:
ECM Package
Operations Center
Overlap with the Legion and OdysseyThe obvious comparison to be made is against the Legion and Odyssey. The Legion fulfills many of the same roles, however, the standard Legion and even the XIV variant are squarely "Low-Tech" bruisers, often reliant on missiles and is true battle carrier. The Aquila is something of an in-betweener that is more of a warship than a carrier. Since it lacks any maneuvering system, the Aquila is more akin to the Astral or Paragon in terms of being unable to disengage. However, whereas the Paragon can hide behind a Fortress Shield and the Astral is (hopefully) not on the front lines to begin with, the Aquila is like the new Champion Cruiser that will have to punch its way out.
Compared to the Odyssey, the Aquila hits harder but is vastly less nimble. Beyond both ships having flight decks, they fulfill vastly different roles. The Odyssey is a true battlecruiser: faster than anything bigger but stronger than anything smaller. The Aquila is a true battleship that happens to have fighters.
OverallI imagine the Aquila being the center-piece of a Midline Battle fleet, Focus Firing on ships already faltering or using the ability to force a strong ship to buckle. As a true battleship, it costs more than an Onslaught or Legion to field (or Conquest, for that matter) but is not as durable as either. Built-in hullmods reinforce that this is a command ship. ECM (if it still exists next patch) will improve the ECM rating and Operations Center will increase Command Point accumulation. Like its smaller cousin, the Eagle, it doesn't have glaring flaws but it doesn't have massive advantages over similarly-sized competitors. It has more rear-defense than traditional Low Tech ships but by no means is it exceptional, especially considering it has no shield coverage back there. Finally, the bulk of its firepower cannot be leveraged to the side so attacks to its flanks will face much less resistance.
Thoughts on this idea and your own recommendations for a Midline Capital are welcome.