I guess I just feel like the last scene seemed suggestive of other possibilities. lain still exists, and meets arisu, who has a vauge feeling of recognition, if I recall correctly. This suggests two things, first lain still is somewhere, and second, she is not totally forgotten.
As such, we do not really know what the end state of the show and the world is. Did lain ever exist? Does she exist now? Is she only alive on the wired? Did the memories really get erased, or did she deliberatley allow arisu to remember?
Making a sequal, a good sequal, would be I fear, very difficult. part of the beauty of the story is that it ends, and that afterward we can imagine all of these possibilities from what we have seen, what taste we have witnessed. Adding a second section risks answering too many questions, which the first part carefully, and rightly, left ambiguous.