We used to close a lot of topics to prevent necro-posting, but recently we switched to letting the customer decide where to post and we'll move any posts in ancient topics to a new topic. Closing a topic can look like shutting a door, which isn't very customer friendly.
Guiding users tot the right existing topic
Another relevant issue we often face is topics that are created, while there is a neat thread with questions and answers about the topic or question the user wants to address. Unfortunately, it's not possible to move an initial post to an existing thread. Are there any other communities facing this problem?
It would be a nice (but probably a pretty advanced) solution if you'd see topic suggestions while you are typing. We could provide 'top suggestable' topics via a moderator tag, furthermore it would just be a search query. The easy way: type a certain word and you get suggestions, the smart way: use AI to suggest topics.