Assuming they have an engine/workflow that lets them freely move assets around, the expansion pack could add a decent amount of content to the sequel.
And, IMO, the 'wild' or 'crazy' direction of the Saints Row series has already gotten stale.
Saints Row 3 had, more or less, the standard sandbox gameplay mechanics of every other open world sandbox game on consoles, like GTA 4, and just added a bunch of weird/crazy storylines, plots, setpieces, items, models, and so on, which made it feel very gimmicky.
They need to add some unique gameplay mechanics, as opposed to the 'standard open world Sandbox' fare of the previous games (More or less.). I suppose one way to do this is to give the player more control, like changing weight/physics/properties of various items, but in a focused UI. Climbing, building, etc, these are some of the things that could set up some truly unique and awesome scenarios in a Saints Row 4 world.