I'm guessing...
2016: remove Excalibur, waterpark expansion that direction + new employee lot, 1-2 flat rides. It is their 40th anniversary after all, so not out of the question to see additions to both the waterpark and the amusement park.
2017-19: A major coaster, ideally something B&M with inversions like a smaller wingrider or dive machine. There's potential for a gate redesign if they removed the underused amphitheatre space and a good bit of the way-too-big parking lot near High Roller. One of these years I bet will be a dark ride as well, and one will be a quiet year like 2015. I would not be shocked to see a western theme. Have they filled the old IMAX lot yet?
The biggest hole the park has right now is the lack of a quality steel coaster with inversions (Corkscrew...). Flat ride selection is not too bad compared to some other parks, but IMO almost every park could use a bunch of quality flat rides. Removing Excalibur for the sake of the waterpark makes sense to me since that corner of the park is so isolated and weird and Excalibur is so mediocre and that's the most logical direction for the waterpark to expand anyway.
Personally I would really love for them to start a shuttle route to MOA Station and maybe the new Golden Triangle Station when SWLRT opens. Mystic Lake has been doing this for years. Currently it's literally impossible to visit Valleyfair without a car. A connection to MOA Station would provide an easy solution for people flying in from out of town (MOA is like 4 stops south of MSP Terminal 2 and the Blue Line goes straight downtown) as well as people like me who live in central Minneapolis and don't own a car.