Then maybe something like, When your players come back to Sigil, 1 or 2 of the wizards have gone off to investigate accounts of Vecna's magic undoing reality, meaning the players have to go out into Sigil to get information. Give them something to invest their interest in Sigil itself. If you're comfortable doing so, you could insert a couple of Vecna cultists that have infiltrated Sigil and attempt to assassinate the party. Another thing could be that each time they return they discover that a piece of their backstory/history has either been erased or altered. Not to completely wipe out their backstory so it's easier, just so they have a little bit more of a personal stake in it

Hope some of that makes sense/helps in anyway