Avatar 2 will be big, no doubt. I do question on whether it's possible for it to be as big as Avatar.
You could bring up the Titanic comparison but there's a couple of points
1.Avatar wasn't as big as Titanic, sure it out-grossed it but that's from all of those 3D prices, plus inflation, plus IMAX.
2.Avatar started the 3D craze, that craze has been dying out, the revolutionary effects were part of the reason it took off.
3.It's a sequel, Avatar was insanely popular, but so was Star Wars and look at the drop from Star Wars to the Empire Strikes Back (and then put those in adjusted numbers. Star Wars adjusts to 1.1b domestic, Empire Strikes back adjusts to 620m domestic for their first runs, absolutely insane numbers for both of them, but it's a huge drop off)
Expect Avatar 2 to be huge, but don't expect Avatar 2 to be Avatar huge.