So my quest for a (desktop) outliner continued after a night sleep. Now I'm done.
If you go to Wikipedia > marktaw.com > Personal project planer you'll eventually end up at an open source application that I haven't been able to find mentioned anywhere else on the Internet.
The application is now called Planz. There is a interesting video on the website.
It remains to be checked out. Hopefully, this is a what Tranglos is looking for.
From FIDO-net, through Usenet, to DonationCoder - the only thing do really miss from those days might be the promising noice from the modem, making a connection.
Anyhow - greetings from IKEA-country, and good luck!