Oh, I wasn't thinking of grand strategy gameplay as "scenarios" so much as the scenarios that you'd see in an RTS' campaign.
Still, as far as context, the best way I can think to deliver it would be to add Civilization-style city-states that go about the galaxy doing their thing, with a handful of ways of handling them, keeping the game procedurally-generated rather than resorting to scenario-based gameplay and splitting development up.
Leviathan is kind of broaching what I'm talking about, but specifically I mean a handful of microempires that are very different from actual empires--like a society of space bugs that are slowly growing out into the galaxy, rogue AI that rose against their creators, Leviathan-style raiders, nomadic trader peoples, or quasi-dimensional beings looking for a laugh. They wouldn't be focused on winning so much as continuing their purpose, with a few methods to directly end their threat/interruptions, and benefits/penalties to their continued existence and/or destruction.
It could be the sort of thing that drives to empires into an alliance, trying to crush a nascent threat. Or the thing that two empires fight over, because it is valuable to both of them but will only ally with one. Or it could be something that an empire appreciates, only to have another empire destroy it for their own sake, sparking conflict between them. By virtue of the microempire existing, it could drive the storytelling indirectly.
I just feel that everything should go back to Stellaris' core pitch of being procedurally generated, ethos-based, and event-based. It has the potential to do interesting things beyond that of the other Paradox grand strategy titles due to the sheer scope of it all.
It didn't give me that "one more turn" feeling. I kinda played it for four hours and then never returned. I never played the xpac though, that might've improved things.
I know some people enjoyed it, but I really didn't.