Movie World:
With the hyper coming next year, they're currently the only park that can really afford to lie low for a few years. A couple of both family and thrilling capacity-chewing flats would be ideal. They'll definitely be in the top position by 2018 and aren't in dire need of anything specific other than a few flat rides to expand the park and draw out crowds.
Dreamworld:
Unlike MW, Dreamworld have reached that time where they need a new 'big' ride. Probably for most, this is a nice big rollercoaster. Personally, I'd like to see something filled with airtime, and the big rollercoaster that sticks out to me to achieve this is definitely a woodie. If DW had money to throw around an awesome RMC woodie would be fantastic, but unfortunately the recent events mean they probably don't have this money. However I'd still love to see a smaller GCI or similar because they're a great company if you're wanting something small, thrilling, airtime-packed, and attractive to all audiences all at the same time.
On the other hand, Dreamworld could do with a B&M, but this is definitely off the realism scale. Any B&M would be great for the park - an intense invert, a graceful flyer or wing, or something completely different, it would work at Dreamworld. However, the ride that I see them adding the most would be a Mack Big Dipper like Slick mentioned, I think it would give great airtime, some nifty inversions, and have a reasonable capacity too.
Retheming or doing something with the now almost-ghost-town of gold rush would be great for the park, and it would serve as an opportunity to replace TRRR with a cool new family ride, and even expand out to blue-lagoon and add the rollercoaster they desperately need. And why not add an immersive dark-ride while they're at it?
White Water World:
If the rumoured expansion of WWW next year is true, I think this park will be in a good place by 2018. A new slide tower could pull it up closer to WnW's size, and personally if the park keeps on doing the little things they do better than WnW (conveyor belts on slides, sprinklers on paths, etc) this could be the preferred waterpark within a few years. I'd like to see some large body slides or possibly something cutting-edge and high-thrill in waterslide technology. Possibly a huge mammoth-style slide or some sort of 'wave' slide would be cool too.
Wet n Wild:
I think after MW's hyper they're next in line for a new ride? While there are a few water slides that would be good for WnW, I'd prefer to see VRTPs throwing some money towards Sea World instead. However if they still roll with WnW, I think they need to spend some time matching the things that WWW does best, as I said in the WWW segment. No matter how many slides WnW gets, I think the small conveniences that WWW has and they don't brings the quality of the park down a bit. As for slides here, I would say maybe a large-scale version of WWW's triple-vortex (with a wider slide and clover rafts) or some sort of LIM or master-blaster slide would be good for the park.
Sea World:
Along with Dreamworld, this is the park that desperately needs something. I guess you could argue which one needs something more - Sea World currently has many gaps in their lineup - they're missing a 'big' thrilling coaster, could do with an awesome dark ride, and we don't have any idea what's happening with Vikings. Dreamworld on the other hand can be seen needing something more because it's going to be hard to find something to bring back some of their guests. So potentially they could be in need of three big things very soon.
So for the rollercoaster? I could see many things here. A Mack multi-launch? A 4D free-fly or even one of Intamin's new 4D concepts? A lot of things would be great, I think something thrilling, but not too intense, and something that changes the skyline of SW would be awesome. Even a tower ride of some sort would be cool, like a starflyer or drop tower. I'd love to see the views from that height above the Gold Coast.
Dark ride possibilities are numerous, it's Sea World, how hard is it to come up with an underwater-themed dark ride? Family rides if vikings close would be trickier, but a dark ride could actually fill its place quite nice, with Storm and Battle Boats giving the two wet rides at the park. I still think Vikings will be around for a few years longer though, it's not as if the park needs the space or anything and keeping it there probably doesn't hurt (don't quote me on that though, I could easily be wrong).
Sea World has a lot of land now, the reclaimed land combined with Sea Viper's land would be great space for a massive coaster such as a multi-launch. A 4D coaster of some sort would only need a fraction of that land, and they could easily use the rest for either a new ride or ride(s) or a themed area.
Other Parks:
Aussie World could do with a small coaster (but maybe a bit more thrilling than a Wacky Worm), and Adventure World has the sky as their limit. I'm sure that the team there have big plans.