Blackbox looks interesting but I also see it is using a cross-platform framework. The GUI might be more natively Haiku than LibreOffice but I’m still concerned that this is not Haiku-specific enough to be a good tutorial piece for teaching Haiku programming. As a bonus, it seems to be written in an Oberon variant and Pascal variant, however.
I personally would prefer the TextView gadget as Paradoxon suggested or a variant thereof since it is Haiku from the ground up. It will also allow more integrated reuse with the OS. If it can be made to link text to another instance of itself as a means of making text flow from one column or page to the next, it would upgrade from a word-processor to a more modular desktop-publishing app.
The pictures and vector art could also use some of the same code as the Haiku icon rendering for an added layer of integration. Of course it would need to embed PNG or JPEG images as well. Is there an ImageView gadget in the OS or on HaikuDepot that would work for that also?