(Hopefully) 2 months of speculation to begin!
This is all Jamberoo has released so far as of yesterday on social media (along with a "#comingsoon"):
Here are some points based on currently known information:
Since Jamberoo is trying to build hype, I'd say this is a 'development' year.
Based on the image's theme, it looks like it is a thrill ride. The 'newly proposed $6.5 million large major custom-designed hybrid world-first ride', as far as I know, has not had a new development application submitted for it yet so for now, it seems unlikely to be that ride. This means it is most likely Thunder Downunder or Emu Mountain Rapids being built next out of the proposed Kangaroo Island attractions as they are the only proposed thrill rides left and they do have approved development applications. But which one would it be? Thunder Downunder or Emu Mountain Rapids?
The original Kangaroo Island expansion timetable was this (rides operating are bolded):
Funnel Web & Banjo's Billabong (2012)
Koala Creek & Platypus River (2014)
The Perfect Storm & Thunder Downunder (2016)
Emu Mountain Rapids (2018)
The actual expansion has occurred as follows though:
Funnel Web & Banjo's Billabong (2012)
The Perfect Storm (2016)
Since Koala Creek is unlikely to be the new addition being promoted as it is not a thrill ride, they could be finishing 'Stage 3' (which currently has become 'Stage 2' due to The Perfect Storm opening), which would mean Thunder Downunder would be built next or they could be going straight to 'Stage 4' which is what they originally planned to have opened in 2018 - that being Emu Mountain Rapids. Due to Emu Mountain Rapids consisting of 2 slides (1 with a Tantrum Alley), I meant in that previous quote "...it is very unlikely that at least half of it (hopefully the Tantrum Alley half) will go ahead now...". But without a DA submitted for the 'newly proposed $6.5 million large major custom-designed hybrid world-first ride', Emu Mountain Rapids is less likely now to have the Tantrum Alley removed. Both Thunder Downunder & Emu Mountain Rapids could be rethemed from it's originally designated themes.
Based on the image, the theme seems to be based on a creature. A creature's eye can be seen in the lower left corner of the image above. Jamberoo particularly focuses on Australian fauna for theming so that may give an idea of which creature it could be. Due to how much of the image the eye takes up, I'm not 100% sure what the creature is though.
Jamberoo's last major addition, The Perfect Storm, was announced very shortly after the first teaser of that ride was released so speculate while you can!
So the main questions for now are:
Is this an 'enhancement' or a 'development'?
If a 'development', which proposed Kangaroo Island attraction is being built - Thunder Downunder or Emu Mountain Rapids?
Are they getting rethemed from their original themes?
If so, what is the theme?