The first thing it needs is another very big city somewhat near it, so it has a large enough customer base to warrant investing in it. Until that happens, it probably won't get anything other than the sustenance minimum.
I wouldn't say that the market isn't there. The park is less than an hour from Grand Rapids, and the entire west coast of Michigan's Lower Peninsula is lined with resort towns and beaches. Families from Southeast Michigan head across the state for a weekend getaway throughout the summer, and Chicagoans tend to travel around the bottom of Lake Michigan to relax on the coast's southern beaches. The market is certainly there.
Michigan's Adventure's problem is that there's too much to do in the area. Beach days, boating, fishing, kayaking, off-roading, camping, bar crawling, museums, art fairs...it's a tall order to ask people to cut the time spent on their other activities in half to go to an amusement park where their biggest draw is a 20 year old wooden coaster.
Now yes, they most certainly could use a significant investment to draw more people in, but there are some pretty big road blocks to that. As Poke mentioned, the park is nowhere near profitable enough to warrant that. I've heard that the city council is not very enthusiastic about amusement parks for various reasons, and thus the park has difficulty getting new projects approved. Finally, I've heard that the park receives plenty of noise complaints from nearby residences and campgrounds, which further muddies any plans for the park to expand.
Tl;dr, they're caught between a rock and a hard place, and won't be receiving any significant investment any time in the foreseeable future.
I mean, sure...add one of the loudest coaster models available to a park that's already receiving noise complaints. Yes, it's a joke, but it doesn't work nearly as well when it immediately follows a post stating the park's noise problems.
Easier said than done. As far as I know, RMC has never filled their track with sand, so they'd have to design in provisions and then the park would have to supply the sand. It all costs extra money that a hardly profitable park doesn't have.
Again, I get the joke, and I realize that there is a solution...it's just not something that this park will do.
A launch coaster would be a good fit. I think, something like a skyrocket II or a triple launched gerst like the one at slagharen could be a good addition to the park. It could be marketable as the first triple launch coaster in the region too.
Well, the park got their first new roller coaster in 20 years. So, that's a start. The last new roller coaster the park received was in 1999. They're screwed.
In reality, the park is very isolated as a tourist attraction. Off the beaten Grand Rapids/Grand Haven path (which is the main tourism draw), visitors struck me as being primarily interested in the water park section, with roller coasters an afterthought. It truly is a bottom-tier park that has just enough relevance to draw a crowd and profit.
It's important to recognize regional markets and anchor cities for parks; which again, Michigan's Adventure really has none. Chicago has Six Flags, Detroit is Cedar Point's number one city. And with only Grand Rapids and East Lansing you're nearby cities, that barely gives you a core population of 300,000. South Bend, one of the next nearest cities, is about equidistant to SFGA.
Population demographics play heavily into decisions that are made by park chains on which parks receive priority projects. And, as is evident, Michigan's Adventure just hasn't been it.
That all being the case
A great, major roller coaster that I could see a good fit would be a Gerstlauer Infinity Coaster, a la Hangtime at Knott's. Great compact footprint, meets capacity draw well for the park, and would give it a novel advertising edge to the market (steepest drop in the midwest?). The park is very U shaped in it's design right now, and it would be great to fit this new coaster behind Wolverine Wildcat, connecting the pathway back towards Thunderhawk to complete the loop:
I think a small-scale Eurofighter like Hydrus or Adrenaline Peak would work wonders here. It has low capacity, a very compact layout, and also it packs in a ton of elements that would be unique to the park. Cedar Fair has worked with Gerstlauer in the past half decade on Hangtime at Knott's, so I think a Euro-Fighter is in the realm of possibility, I just don't know when this will happen.