Fairly simple feature request:
There are a few actions you can do to multiple topics at once: close, delete, move category, change tags, etc. It would be nice if merging topics could also be done through that interface, rather than having to go to each topic, select all the posts, and move them to one topic.
Manually moving like that has the advantage that you can move old posts to a newer topic. For a bulk action like this it would be reasonable to just move all the posts to the oldest topic.