I would have said lower in most circumstances, for the same reason that Star Wars saw a drop, even though Empire Strikes Back is considered the pinnacle of the series.
However, there is going to be such a rapid expansion of 3D theatres that there will be at least twice as many domestically, and who knows how many times more overseas. This will ameliorate a drop, and even better - the degree to which 3D will be expansing across the globe could possible give Avatar 2 an increase over Avatar.
There is one more factor for a worldwide increase. Emerging markets, such as Russia and China, are only just starting to see good sized grosses. And they are accelerating at a ridiculous pace. Avatar's $100m in Russia could be equivalent to $200m in 4 years' time, simply due to such a huge market expansion. This will affect all movies, not just Avatar.
So, for the above reasons, I think Avatar will increase. I assume Avatar will make $750m domestically and $2.25b overseas, for $3b worldwide. So, these are my predictions for Avatar 2:
DOM OW: $190m
DOM total: $825m (+10% over Avatar)
OS OW: $610m
OS total: $3.175b (+ 41% over Avatar)
WW OW: $800m
WW total: $4b (+33% over Avatar)
