Fediverse

I kind of don't necessarily want another locked in non-federated site to replace reddit when we have lemmy and other potentially better / less locked in options.

That said, I'm not against standalone forums, so tildes.net ... well, it looks like a currently less successful lemmy instance with about the same or less engagement, and I can't see how one would sign up if one wanted to. Last I heard there was an invite process? No thanks really.

You also need to really define "reddit alternative" - do you mean a forum? Or something trying like lemmy to be a sort of clone? In the "forum" link aggregation option, there's HackerNews. There's the BBS The Well. There's sort of internet service provider focused forums like dslreports.com.