With coronavirus now impacting basically every place on the earth, and anywhere not significantly impacted trying to keep it that way, pretty much all non-essential work that requires large groups of people has completely stopped. That includes most if not all film productions. What the long term impact of these delays will be is unclear, mostly because we're all still unsure when work might resume, but one has to assume that it could mean delays in the release of a number of upcoming films, including some films that we've already been waiting on for quite a long time, like the Avatar sequels.
Work is, or, in this case, was, ongoing in New Zealand to film Avatar 2, 3, and, according to Variety, parts of Avatar 4 as well. However, that production, as with so many others, is now on hold. The film's executive team, that may or may not have included director James Cameron, was set to fly to New Zealand last week in order to resume some filming, but that trip was obviously cancelled.
Even if getting the production underway was a good idea, and most would agree it was not, New Zealand currently has some significant travel restrictions in place, including a 14-day quarantine period for any international travelers entering the country, so the Avatar team would have to wait around for two weeks before anything else could even be done. There are a dozen confirmed cases of coronavirus in New Zealand, though, thus far, no deaths.
Avatar 2 is currently scheduled to release in December 2021, 12 years, almost to the day, after the first film debuted and became, at the time, the highest grossing film ever. Fans have been waiting for a long time for these films, and there's every reason to believe they may have to wait a bit longer. All the Avatar filming for all the films is happening at once, rather than sequentially, so there may still be shots for Avatar 2, that are left to film.
While filming has ceased, it's unclear what's happening with post-production work, which is being done alongside filming. That might still be going on, or it may be happening on a smaller scale with a smaller team, or it may have also stopped entirely. Even if filming for the first sequel is complete, post-production almost certainly is not.
If there is still a significant amount of work to be done on Avatar 2, then the longer this stoppage continues the more likely a delay will take place in the release. Even though Avatar 2 is nearly two years away, it's not outside the realm of possibility that the coronvirus today could still be impacting the world two years from now, even if the virus itself is no longer an active concern.
Of course, with so many other movies impacted, and release dates being pushed back, Avatar 2 could end up seeing a delay not because it needs one, but because Disney will have a collection of other films that need new release dates following the mass production shutdown. Expect to see a massive shift to the release calendar once the dust has settled.