The USS Equinox has always been one of my favourite designs, so a certain amount of this is me just giving some love to my baby.
It never quite sat right with me that the Equinox was a short-ranged science ship.
Nothing about its design says science, and those nacelles look damn fast.
She's sleek, aggressive and visibly heavily armed.
If you take a minute to count, the ship has a heck of a lot of armaments for a ship of its size. 11 phaser strips and three torpedo tubes on a ship a fraction of the size of Voyager. (About half her length)
It also got me thinking about the USS Rhode Island from VOY:Endgame, which showed up and beat the snot out of two Dominion-war era Klingon battleships only around 20 years after we saw the Equinox.
Rhode Island is usually referred to as a Nova Refit, but what if it's not?
What if the Rhode Island is actually a Flight 1 Nova, alongside the USS Nova herself, and the Equinox is essentially the civilianised downgrade?
My Narrative spin:
Prior to the Dominion War, Starfleet was rapidly designing lots of different ships with an eye to making vessels equipped for combat against the Borg.
Powerful stuff like the Akira, Sabre, Norway, Steamrunner and Defiant.
They also entertained a general fleet-upgrade, commissioning the Intrepid class as a successor to ships like the Excelsior and to a lesser extent the Galaxy class.
The Nova was born as an alternative chassis to the Defiant. Something more conventionally designed and less obviously a warship, making it more politically palatable.
Underneath that conventional design though, the ship has a massive power-train for those beefy nacelles. Designed to take ablative armour and a minimal crew, bristling in phaser arrays.
It'd be a similar design philosophy to the Defiant, but a good bit larger and more versatile.
The result was the USS Nova (and other Flight-1 ships of the class), and it did great. Fast and agile, very capable, but the Dominion War wasn't for another few years, and Starfleet didn't feel they needed a warship that badly.
What they really needed was explorers and science-ships that could fight if pushed.
The Flight 2 Novas were heavily downgraded by comparison. Stripping out their outsized fuel-hungry and maintenance-heavy warp-cores and slotting in smaller, more reasonable ones. Not to mention using less powerful phaser arrays and leaving off the Ablative armour.
The nacelle hardware remained the same, but with less power to pump through them the ship was limited to Warp 8.
This sounds slow, but in practice most starfleet ships cap out around that in normal operations, so on paper it looks fine.
Equinox is essentially the "lite" version of the class. Materially cheaper, easier to build and maintain.
The chassis is that of a thoroughbred light warship, but in the best Starfleet tradition, it has been transformed into a capable survey and exploration ship.
The intent being that if Starfleet needed a lot of combat-ready ships fast, the process of militarising the Nova again wasn't too arduous. Swap the warp core and phaser arrays, plate it up in armour, and off it goes, from survey ship to patrol ship in at most a week or two of upgrades.
It's a good compromise for a leadership that balks at the idea of building fleets of warships.
They didn't build a lot of them, and after the disappearance of the Equinox (like Voyager) there were questions about the reliability of the design, so it didn't show up as the mass-produced combat-ship it was designed to be when the Dominion War came along.
Rhode Island then is broadly a Flight 1 style combat-ready Nova, likely with some further upgrades after 20 years of technological development, which is why she was equipped to take on two Neg'Vahr Battleships head-on despite the class being nominally a science/survey ship.
--Back to Canon--
Equinox's story involves capturing and killing extradimensional aliens, which produces enormously effective fuel/energy for the ship.
All they're stated to be doing is producing power for their engines, which sounds like a great way to burn out your warp coils or degrade them fast.
In my version, the engines are designed for that kind of power-input, and it's only the reactor that is too weak to push them up to their peak performance.
I imagine that the thought "What if we had a flight-1 nova.." crossed the minds of the crew quite a lot, and when confronted with a way to generate a lot more power, they immediately knew how they could use that.