We've been talking about this with Cedar Point for years, they're running out of room for major additions.
As we stand now, they have the space where Shoot the Rapids used to stand and the Wicked Twister area, which has been rumored to go along with Windseeker for a new addition as the stadium was just torn out. Outside of that, they would have to take out something already standing or renovate. Hell, look at their last few coasters: Mean Streak and Mantis renovated, Valravn on vacant land where a coaster once stood (Wildcat) as well as taking out another older attraction, and Gatekeeper.. same as Valravn. Outside of vacant land, there would have to be something removed. What more could be done with what they already have that would be major? The options are very limited. Thats why I favor a smaller addition instead of something major because logically, it just works. Yeah, I'd love a 400ft TRex.. but lets be honest with ourselves..
Indeed, you can say in more than one way that Cedar Point is running out of opportunities for new big things, both in terms of adding something they don't have already, and in the space available for new development. I don't think either factor is a guaranteed show-stopper, but they would have to think creatively to overcome them. Finding something new to fill the lineup with seems like the least of their issues, so let's consider the space problem first. I haven't been there in person, but looking at the map there appears to be three zones where they could theoretically expand with something big.
First, the main parking lot. Open-air parking lots are notoriously inefficient when it comes to land use, and at Cedar Point the land is at a premium. It could be theoretically possible to free up significant portions of the parking lot by building multi-storey indoor parking instead. The parking lot covers an area half as big as the park itself. Cutting just 30 % off it would free up more room than any coaster could realistically required. The main challenge would be to connect a ride there to the rest of the park, as it would have to go around Gatekeeper and Blue Streak somehow.
The second would be the Soak City car park. Same story there, it's three times the size of Gemini's footprint, all sprawled over open ground. A garage there would free up more than enough land for a new big coaster, but once again it would run into the problem of connecting it to the rest of the park. Gemini is in the way, and we may assume that retiring it is not an option.
Third, the island with the defunct Shoot the Rapids and Millennium Force's turnaround. The former could be demolished, the latter could be built around. If Blackpool Pleasure Beach could fit Icon in between their existing rides, Cedar Point could fit a coaster around the turnaround. The land is accessible from the park as well, as demonstrated by Shoot the Rapids' existence. However, utilizing this space would require a lot of trees to be chopped down, and trees aren't really in abundance in that park. So overall, there's room if they want it, but they'd have to compromise to make it available.
Besides, it's not like a memorable addition would have to be terrifyingly huge by necessity. What would excite me the most would be something in the spirit of Maverick. A quality coaster that's only large-ish by size, and which just provides a really thrilling ride without trying to break any records. Cedar Points lacks a good sit-down multi-looper, so there is a niche for it. And not to beat the long-since-dead Aquatrax horse, but that ride type is a water ride which doesn't necessarily involve soaking the riders. If they could somehow combine a water ride, a coaster, and a really great ride experience, they could provide an attraction worthy of a 150 year anniversary while staying below 30 meters in height and a kilometer in length. It could feasibly even fit at the Wicked Twister/stadium site - although I suspect that area is reserved for a kiddie/family ride like you suggest, given its proximity to the kiddie area. Question is if that will be in 2020 or another year, though.
