Honestly, there's no real need for scenarios. You can literally just do all of them on your own, and you'd actually have more wiggle room for personalization if you do. The only unique ones are the ones that create specific character dynamics (having two sims be exes or enemies), and even that can easily be replicated in-game. Scenarios seem inherently redundant, and so far, nothing has shown me otherwise.
I'd like scenarios that actually have some unique impact. Like one where your sims must live entirely underground for a certain amount of days because... radiation on the surface, I guess? And the game won't let you leave your bunker, which you can expand and edit as you please. Or maybe you play as a thief and can get jobs stealing certain items from certain sims. Or something with a storyline and characters. Again, these could be done without scenarios already. But they'd be very unique circumstances and it would be cool to see the game enforce them.
There is ONE idea, though: dragons. Think about it: you patch them into the basegame, and you can start a scenario where you're given a dragon egg and have to raise it. Again, nothing groundbreaking. But it'd serve as a good tutorial for the new gameplay.