National Lampoon's: Australian Vacation
Assuming such a film came out in 1987, a year after the hit Crocodile Dundee, heck perhaps even set back to 1988, just so they can learn from Crocodile Dundee and perhaps rewrite some gags to avoid appearing to plagiarize and look similar etc.
What gags would you have put in? Remember to think of the late 80's!
I would have the Grisolds plan a big Australian trip, rather than say winning another game show.
Have them land in Sydney, touring the sites of course, causing havoc somewhat in their rental car of course (with Clark still not getting used to the idea of left side driving, roundabouts etc)
Perhaps a stop over at the nation's capital Canberra. Maybe an international incident with the Prime Minister.
They head down south, to Melbourne, where one gag could be the difficulty for out of state peeps to do a "Hook Turn", only after some frustration by other drivers involved that they realize that the Griswolds aren't from New South Wales/Sydney but are tourists.
Have them go east, towards Perth, where maybe one of Ellen's relatives live (which has a stop over in Adelaide and is a perfect excuse to go west)
We head on by road north heading to Darwin. At some point the rental car breaks down for good and the replacement car, which happens to be non other than a Wagon Queen Family Truckster (modified for the Australian road where the steering wheel is on the right) is provided as a replacement by the nationwide fictional car rental company).
In Darwin we perhaps catch a glimpse of a Mick Dundee parody, someone who claims to be a crocodile hunter of sorts.
We then head to Brisbane, why? No, not just round up mainland Australia's capitals, but to attend non other than Expo 88! At this point the Griswolds hear about a theme park called "Wally World Australia" located at nearby Gold Coast. The climax of the film could have been them visiting the Australian version of the beloved American park, where Dreamworld would have been modified to reflect the fictional park.
What do you think? Any more ideas?