I think the killer app would be to keep the community somewhat exclusive but also have a great breadth of niche, expert content. There was a time when Reddit hit this sweet spot (although there were also very large subs that attracted the worst users).

One of the main issues with reddit is that a lot of niche interest subreddits are 90-99% non-practitioners or extreme amateurs, even when weighted in terms of contribution. For example there is no subreddit where you can actually learn very insightful things about programming or computer science or machine learning, same for biology/physics/astronomy/etc.

I would be ok with a not-reddit competitor that actually succeeded in being a better reddit. But the #1 thing they should avoid at all costs is taking VC money. You can't have a great community of experts that is also trying to maximize it's viewership, that's how you get Quora or Reddit