There are already quiet a few softwares that claim to be Notion alternatives or seem to be:

- AppFlowy: https://github.com/AppFlowy-IO/AppFlowy

- AFFiNE: https://github.com/toeverything/AFFiNE

- SiYuan: https://github.com/siyuan-note/siyuan

- Trillium Next: https://github.com/TriliumNext/Notes

- AnyType (only clients are source available): https://github.com/anyproto/anytype-ts