Something I would definitely pay for is a whiteboard app/layout. I have found Cecily which is something similar but development seems to have been discontinue a long time ago.
I know of some new whiteboard apps such as Heptabase and Scrintal but these are proprietary. Another one is Kosmik which seems very promising and the developers are planning to open-source it.
However, I don’t know if any of these are going to achieve the level of ‘‘interconnectedness’’ and customisability TW is capable of. If there is interest in the community and an experienced developer would be willing to take this on, this could be another Opencollective project similar to the Fileuploads plugin.