I think a big issue is that any of the big contributors of the past, lukeprog, EY, Yvain, gwern, Kaj_Sotala etc. aren't writing articles here anymore and there is no other similarly good and popular writer that would do the same today. There is no purpose coming here, except for the Open Threads. Posting and making comments itself is not very fun because you always have to watch out what you say.

Anything that requires many people to change their habits probably won't going to happen. Changing norms is difficult for the same reason, so idea D is possible, but a bit hard.

I think a combination of

a more co-operative and cheerful style of discussion

and

allowing and encouraging submissions of rationalism, artificial intelligence, transhumanism etc related articles from elsewhere, possibly as a separate category.

and generally allowing a more free discussion about related "fun" issues

could work, but I don't know how would you go about implementing the first one. Like I said, changing norms and habits is a bit difficult and making announcement to the effect of "Be more co-operative and cheerful" is probably going to work as well as announcing "Be nice and don't bully!" in schools. Just relaxing moderation wouldn't be enough. Because honestly, I haven't even noticed any moderation here, there's just the feeling of it.

There's the possibility of starting from scratch and making the subreddit /r/lesswrong more active. Then changing habits and community norms wouldn't be such a big problem. LW related subreddits like /r/rational and /r/HPMOR are already pretty active so expanding this "LW cluster" on reddit would only be natural progression. But I feel like even if that happened, it wouldn't be a satisfying solution for many.