Wouldn't get my hopes up for 2020 yet. Especially for a coaster. Probably won't be until 2022 till they start really working towards the next coaster, unless they finally get the long speculated family coaster before then. I don't know that HFEC's relationship with Mack is that good at this point anyway... have you seen how much downtime TT has had? Yes it's popular, but it has to be very frustrating to own and operate.
I think the plausible speculation running around right now has to do with "revitalizing" or replacing the classics (Flooded Mine and Fire-In-The-Hole), or coming up with a new water ride. The park has three water rides right now, and of those three, two are seriously antiquated and don't have the through-put they need at this point. Riverblast just simply isn't that fun or popular. Since AP and LR are so old and have their issues, it would make sense to get rid of one and get a shiny new replacement. Personally I'd rather see AP replaced with a modern flume similar to DW's.
We know that Flooded Mine is going to get shut down or completely re-done soon though. The park simply hasn't put the money into it to keep it going well, and at this point it is getting away from them. I'm still a little upset that they haven't really mentioned the 50th anniversary of the ride, other than allowing the operators to put up a little sign. It should be a big deal, but they don't seem to want to draw attention to it...