@SarkasmusTLDR: I am not a lawyer, but:
1) Lazeez has indicated that he will publish story continuations and is (in general) not concerned with potential copyright issues; and
2) Continuations are specifically allowed under WLPC's author agreement in some circumstances, and
3) The legal state of a continuation is probably 'Fanfiction', which is probably covered by fair use.
(Apologies in advance if that's not the perfect summary of Lazeez' position - plus, of course, it's from 2019, and things might have changed in between, but I haven't seen any indication of that)
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Lazeez has posted on this. His comment is (in part, there are many) is:
What I do, when asked if somebody can continue a story is to tell them to post their continuation under their own name and when they do, I add a link at the end of the original story to the continuation making it clear that it's a different person. If multiple authors want to finish it, then I link from the end of the story to the multiple endings.
See Adding to a late Authors stories
Storiesonline / WLPC's Author Agreement contains this:
16. If you don't log into Storiesonline for more than 3 years, your stories will get moved into the archives which are accessible to premier members only. We may give interested authors permission to finish any work you left unfinished.
That allows Lazeez to grant permission (obviously, it doesn't require that, or grant it by itself) to continue a story should the author not log in. In the case of a 2007 story, that may be the case (or it may not - depends on the author).
If the author is still active, the new story is essentially 'fanfiction.' As of now, the state of the law on fanfiction is largely unknown, but a continuation (or, even more distinct, an 'inspired by' story) would (IMHO, IANAL) likely fall under 'fair use': it's transformative, the current work is non-commercial, it has no effect on the commercial value of the work (which is currently effectively $0), and the new use is also (presumably) non-commercial. One does not need to meet all parts of the fair use test to qualify, but this covers three of the four (the other is how much of the work one uses, which in this case would be 'all').