A good idea. There has also been some discussion of making the reply button a sort of GMail style thing where you can change the “type” of reply, e.g.
However, changing the type of reply to topic is a bit of a radical change, a bit more like editing the title in a GMail reply chain, which is quite difficult by design.
I am not sure Discourse needs that level of complexity at the moment.
Yes of course.
And flagging extended, off-topic digressions (that someone clicking on the title would be unpleasantly surprised to find) is certainly something we would like the community to do. You should be as annoyed as us when someone derails a topic, I don’t want it swept under the “oh that’s just the way things are here in broken-down threading town” rug.