Board Thread:General Discussion/@comment-71.60.196.148-20160124042613/@comment-27384850-20160526153248

The next season will be Alicization as that is the next arc in the books, however the format is unknown. The two most likely scenarios are that it is broken up into two full-length seasons or is aired as one incredibly long season like FMA: Brotherhood was. These are not the only options though, it could just as easily be made into four half-length seasons or anything else. For all we know it may require more than two full-length seasons to complete.

As for Alicization being the end, there's no way to know. It was the end of the web novel, but Kawahara has said that he'd like to keep going with the series if he can. It's up in the air at this point when the series will end.