It is very unlikely that we’d build threads into topics any time soon, we’ve said no to this many times and flat discussions are one of the earliest design decisions that went into building Discourse: Web Discussions: Flat by Design
Topics that have many sub-discussions are generally hard to follow with or without nesting, and if there’s anything important nested within, it can be very hard to find on repeat visits. If a topic is starting to branch off in different ways it usually makes sense to branch off into a whole new topic instead.