Well you're supposed to read the whole thread and then reply, you're not supposed to read half of it, reply to that and then read the rest of the thread. But you could always bookmark the post you're replying to I suppose?
It's a good idea about different types of opening posts, though. I like the conversation element of MN and I like the fact every post stands on its own. I don't think it's a problem that you can't easily see who is talking to who. You often can now anyway with the quote function.
I did also wonder if the save button was going to morph into a like-by-stealth. I wouldn't mind being able to react to posts with emojis, that's a bit different to the idea of a like option but is a bit more varied. Might be a recipe for disaster with people doing passive aggressive reacts, but could be fun too.
If you had different types of thread, you could do more complicated polls, which would be useful, you could have "canvass opinion" type threads and responses could be clustered by whether they contain the same keywords maybe in interactive bubbles showing which keywords show up most often in responses.
I'm not fussed about nested threads, mostly they are annoying, sometimes useful, but in general I find them fiddly. I don't think MN has to be like every other site.