I have a feeling the announcement for next year's attraction will be coming either 1) before summer yet after the Mako has its final announcement or 2) after summer but before HHN marketing picks up.
I think next year's attraction will be our version of the SeaWorld Rescue rides. The truck one to be more specific, and I think it will go either where the Terrace BBQ is or where Wild Arctic is. For the the following major attraction in 2018, I would put money on either a new show or a dark ride on the scale of Dark Kastle at BGW. The 2019 minor will probably be a small yet significant renovation of a current ride like Kraken, Atlantis, or an animal exhibit. Lastly, if Blue World still happens, then I would put money on that being the 2020 new major attraction.
On another front, an Asia, Middle East, or even South American park may be in the future if they want to have a fallback in case another Blackfish disaster occurs.