I'm probably a niche within a niche market here, but the apps I'd like to see would be:

Mathematical plotting - you can do this using the Droid48 calculator emulator, but the result is in black and white and small. It would be nice to be able to do it in color and take advantage of the larger screen (and be able to save the plots, which Droid48 doesn't allow).

LaTeX compiler - this is open source so you probably couldn't charge for it, but it would be nice to be able to alter LaTeX documents on the Edge rather than having to use my laptop, as most of the stuff I'm looking at on the ereader side is PDFs generated (and regenerated...and regenerated again...and...) from LaTeX.

Screenshot capability - this would solve the above problem of not being able to save graphics.

Image editing - Sketcher is decent, but doesn't have all the tools I would like, probably since it's designed for a much smaller screen where such an app would be useless.