Shameless plug: for bidirectional linking, check out Obsidian! We're in private beta: https://obsidian.md

Spaced repetition might also be supported in the future, but it's not as high priority as links. We also have a graph view for link relationship. Oh, and one thing different from Roam is that Obsidian reads your local folder, so you don't need to put stuff in the cloud, and you own your data 100% (if that's your thing).

Yes. I've heard people who see this tool as an outliner tool want that feature. But to be frank, the priority is low, so implementing that feature will be pretty late.