I recently watched Andrew & Danny's COASTER-net Uncut episode regarding the future of Cedar Point. I'm on the same page, I believe 2020 with be the 20th (as you stated, a thing with numbers).
We're already looking at 2015 with the Rougarou conversion and all the other park changes. While, no doubt, this cost some money, it still allows room in the budget plus the upcoming returns to spend on a big ride in 2016, 2018 and 2020 (which, if each of those is coasters, will make it to 20 seeing as the park counts Pipe Scream). We see that the park (and chain overall) is not afraid to make changes seeing as this conversion is taking place. While the skating show was popular, the theater had some structural issues and, seeing as it sits on prime real estate, it was a smarter idea to remove it Now we got that open land sitting there.
With that said, the moving of the rides that were there (and the removing of Turnpike Cars) shows that the area is being prepped for use soon and not way down the road (2020). I don't think that the park would do another renovation (a la RMC doing Mean Streak) immediately after the Mantis conversion.
I think we'll see construction start during the season and an announcement in August regarding that leaked Dive Machine. That's a lot of room for a Dive Machine and there will still be room left after the construction. With the Cedars rumored to be removed following this season, there is tons of room. The coaster could be a major marketing tool for the park. Only the third Dive Machine in the US, first in the Midwest yada yada (what you'd come to expect to be listed). However, it would also set records like the only park with 5 coasters over 200 feet, tallest, longest, fastest, most inverted Dive Machine (etc....). However, correct me if I'm wrong....but at 223 feet, wouldn't it be the tallest coaster with inversions? Reaching 80 mph, wouldn't it be the fastest? I know I'm listing off random records, but you know they would claim it under the records list. I also believe, seeing as a "loop" was mentioned in the "leaked plans", with a speed of 80 miles per hour, it could definitely complete the worlds tallest loop! I'm not sure if they'll go for tallet inversion all together and take it from GateKeeper but I could see that record coming along. Sorry for rambling on about that, but you know that, if there's a record to be claimed after a coaster is built, Cedar Point will claim it.
The Dive Coaster would bring us to #18 (including Pipe Scream like the park does). Going into 2017, I wouldn't be surprised to see a Mean Streak renovation happen at this time. Mantis got more riders than Mean Streak did and that coaster ended up facing a renovation (which will not only improve ridership, but shorten the weight times with quicker dispatch AND three trains). So I think Mean Streak will face a redo. It'll probably get the Iron Horse treatment and become a great ride again. However, this would lower Cedar Point down to just one wooden coaster. I could also see a waterpark expansion occurring here and the rides that "survive" in challenge park (Might only be RipCord) being moved into the gates to get more people to ride them (SlingShot makes mad bank). SkyScraper may make the move but I think we'll be saying goodbye to mini-golf and the Go-Karts. The Go-Karts have been biting the dust at most of the other Cedar Fair parks and that Mini-Golf course isn't in the best shape!
In 2018, I could see that dark ride/coaster coming. It'll probably be the first coaster since Maverick that wasn't a B&M. However, this all depends on where it goes. I could see it NOT being a coaster if it went in in the Pirate Ride location and just expanded a bit. However, it also could be. At this point, I'll say it will be and it'll be in that area. Just thinking because, after Disaster Transport closed, they said they were "looking for a ride to fill that void". That could just mean a dark ride, but I took it as a dark ride/coaster combo. Take it as you will. That could bring us to #19.
In 2019, getting ready for the big year. I could see some new kiddie rides added to the kid areas. I so badly want to say that a B&M Family Coaster would appear, but that would make us reach #20 prematurely (if we really think the 20th will be in 2020).
2020 will be a big year. The parks 150th, Millennium Force will be turning 20 etc... Many speculate that this is the year that a big record breaker will be added. I, for some reason, feel like it'll be a wooden coaster. Get the wooden count back up to 2 and it celebrates the past while also embracing the future. Whether its a good GCI or a monster RMC, I just have a feeling the 20th will be a wooden coaster (as long as Mean Streak gets the conversion to steel. If it can be saved for wood, that'd be awesome, but that'd cost a lot more in the long run I believe wouldn't it?).
Anyway, just my speculation and, of course, these can happen in any order. But I am excited to see what the next 5 years will bring. I may be moving out of the area in the next year or so, but I will definitely be coming back to the park yearly. Heck, if the Dive Machine stuff ends up being true, I am excited for it!