My need that I want to fill is a bit unrealistic and unfair to expect but... everything.
What made Reddit slowly morph from just another interesting webpage for amusement to a place where I spend a lot of my time and rely upon for so many things was how it slowly came to intersect with everything.
It became a kind of a separate google when you didn't get much joy from traditional web searching. It was a place to feel one belonged but at the same time a place for anonymity when I wanted it (at least to other Redditors anyway), a place for serious discussion and pointless shitposting seeing news as it unfolded but also stupid cat videos.
It was a placeholder for every niche you could think of so if you were trying out a new hobby, or watching a new show, or starting a new career, or trying new software there was always a sub for it.
Lemmy and other alternatives theoretically could do that, but, it'd be hard. Reddit couldn't really do that because of a great design, it just naturally progressed that way when it had more and more people in one place. That centralisation was it's flaw and it's strength so it's a difficult line for any would be successor to straddle. Ultimately though I think, if nothing ever does pull that off, Reddit ultimately created the need for Reddit and we were all fine before it and should be fine after.