Broadside vessels could serve a fleet support and true Escort role. Escorts currently behave more like Rogues by hiding, backstabbing, retreating quickly, etc. A fleet support vessel would improve the survivability of larger, more vulnerable, vessels. They could clear enemy fighters, incoming projectiles, and harry fast moving Escorts.
The addition of a Broadside class could lead to a more symbiotic relationship between ships in PvP. Perhaps as T6 dawns and more impressive capital ships are released, they will need additional support. If players can keep their big warship alive for longer, then it can do more damage to the enemy fleet.
The general attributes of the ship would be something like a light Cruiser or heavy Escort. They would need to be somewhat nimble and fast, but not fighter like. They would need passable shields and hull, but the ships are designed to protect a fleet, not soak damage.
Weapon Firing Ars (direct image link)
This is the interesting part. I am proposing 9 weapon slots, as opposed to the typical 8. Science ships, and some Escorts, have 6 or 7 weapon slots. Some ships have more, or fewer, consoles. I believe that based on the weapon restrictions, that 9 slots will make sense.
Weapons should be restricted on the broadside arcs. No regular Phaser Arrarys. This will prevent overlapping of the broadside arcs and substantially reduce the maximum number of weapons that can be brought to bear on a single target. See the linked image above for examples with the various weapons equipped.
It might be possible to equip all turrets and fire 9 weapons at once, but that is just turrets. The damage falls off with range and the reduced maneuverability of the Broadside class could mitigate the utility of 9 turrets. If this really is a problem, turrets could be restricted to only the fore and aft slots.
Due to the additional weapon slot the Broadside class could possibly have reduced consoles. Do keep in mind that the firing arcs are structured to reduce overlap, so these ships will do substantially less direct DPS than many others.
Science ships get Subsystem Targeting and Sensor Analysis. This helps mitigate their fewer weapon slots of the class and reliance on tight arcs for the Science powers. The Broadside class would need an ability or two that could help them support larger fleets. Perhaps an improved BFaW like power that works with Canons and Dual Beam Banks. Maybe another power could be an improved Transfer Shield Strength type power that represents a deflector dish that is tweaked for support.
I believe that having some unique abilities based on the Broadside class will help the ship fill its role and keep it from just being a slightly different Cruiser or Escort.
Faction diversity can really help spice up PvP and provide reasons to play the other faction. I do not propose completely locking the Broadside vessel, but perhaps one faction could specialize in them. As always with me, it comes down to the Romulans. There are few canon examples of Romulan ships. The ones we see the most of are the largest warships or the smallest shuttles. I propose that to give a new faction some flavor, the Romulans could specialize in Broadsie ships. Soft canon sources often refer to large Romulan fleets that dominate the Beta quadrant. They certainly would have other ship designs beside the Warbird. In a post-Romulus time, it may make sense to produce smaller support ships, as ever large ship would be a very valuable asset.
Whatever happens, I would like to see the typical Cruiser, Science, Escort tree shaken up a bit. I think that there are many new ideas that can be brought into STO, while keeping the theme and spirit of the game intact. There will come a point when no matter what the skin of the ship, it will just be a minor variation on an existing layout. Adding this distinctly different class could prevent us from running out of ideas any time soon.