As seen with numerous other additions at CGA itself as well as the rest of Californian parks, they have to deal with a ton of red tape to gain all the necessary approvals to build anything. That is almost certainly why the slide complex is going up in August instead of January.
Regarding Michigan's Adventure, a water park expansion doesn't quite seem right because, even though the cars are adjacent to the water park, it would be awkward to expand in that direction. It would require the park to remove some buildings and would also situate Zach's Zoomer behind whatever gets built in that plot. As previously mentioned, a kids area makes much more sense, considering the restrictions surrounding the park and the target audience.
It’s a slide complex. They can literally go up in about a month, it won’t take that long. Why CGA started construction now when most parks start in January of the next year is beyond me.
The only reason I can think of for Great America starting now is because their expansion is more than just a slide, but even then, neither Cedar Point nor Carowinds started this early and they both opened on time.