Yet another series idea I doubt anyone would go for… Remember the Transport/Tug from the old Tech Manual, swapping out different types of modules for different types of cargo, passengers, etc, yet still a significantly-sized Starfleet ship? I like the idea of a series following something like that: a big Starfleet ship whose main mission is bringing different types of big important cargoes and passenger/colonist groups across the Federation, getting drawn in to drama relevant to them, fighting off raiders trying to pirate them, getting into trouble at the loading ports and the destination worlds, etc. Ships like this are probably the backbone of Federation commerce. You could combine “South Sea freighter adventure in space” stories with traditional Starfleet drama.