According to Emmerich himself (via EW), the scripts for both Independence Day Forever 1 and 2 are complete, and will play out… pretty much exactly how they’d have to and how you would expect.
”The humans knew that one day the aliens would come back… And they know that the only way you can really travel in space is through wormholes. So for the aliens, it could take two or three weeks, but for us that’s 20 or 25 years… It’s a changed world. It’s like parallel history. We have harnessed all this alien technology. We don’t know how to duplicate it because it’s organically-grown technology, but we know how to take an antigravity device and put it in a human airplane.”
The biggest variable remaining is who will return and who won’t- Pullman is locked, Will Smith ain’t, and no mention of Goldblum yet (that I’ve seen). The script apparently brings in a lot of new, younger characters to fill things out, but there’s going to be heavy lack of charm to this whole thing if all we’ve got is one returning cast member.
There’s no projected start date yet, as another rewrite is going down while the concept artists start cooking up a parallel, post-first contact/conflict Earth. Emmerich has also already spilled the beans that we’ll kind of win at the end of the second film, but then shit will get real in the third (before we win again because, you know, of course).