While I was thinking about the QF (quick-fire) guns I proposed for smaller ships , I pondered how they might be used if they were an option when a ship was first designed. Transports are an obvious choice, as they're a great defensive weapon. What else then? System ships would probably make use of them, again as largely defensive weapons. They're very good against smaller ships like raiders.
But finally, they'd make a great weapons for smaller system ships, like fast interceptors. Traffic cops, revenue and customs, border patrol. That sorta group. I'm sure the Imperium has someone that fills that role, and these would be the ideal ships for them.
Mind you, they'd probably pass for Space Marine ships too, but they'd need an option pack for a slightly different loadout.
Trying to keep away from actual GW IP, but it's not too difficult due to every GW ship being fat. The gun is a QF twin or quad barrel, which would be separate, possibly metal either stock or turned. The gun assembly is already separate from the ship, so I can easily make an attachment point and a number of gun options, for those who favour a more SM type ship.
Still undecided on the prow, aiming for something more cobra-ish than anything else in the IN fleet, but a split-prow might be doable, perhaps something with launch bays?