One modification I'm doing with my group that's working *wonders* for the over-arching ticking-time-bomb effect is I'm spreading the Pall across the world once the citadel returns. 5-7 days per global alignment shift. Just a few days in and all the pure good hearts in the world have been corrupted, and just a scant few hours all the neutral hearts will turn evil (further restricting which replacement heroes can enter the campaign). It's really making them stop and think, "Do we really need to rest? I mean, really?"
I also setup the Architect's area to be filled with an anti-magic field that could be turned off via lever so they would have to deal with a couple challenges in a mundane way (no +5 cloak of resistances for you!), and that was a quite fun and memorable (difficult in the moment) modification.
They have yet to find the method of stopping/reversing the pall, but building materials have been produced at exceptionally high velocities and infighting has begun already. This will be a campaign they will speak of in years to come.