It's probably better to submit your suggestions to Cerious by e-mail, as they don't visit here often (it's a user-to-user forum).
I don't know if you're looking for a work-around, but what you can do is..:
[*]Select your images for the slideshow
[*]Add them to a gallery (right-click on a selected image and pick 'Gallery|Create' from the context menu). As sorting order, choose 'Orientation'. The images will be sorted from very vertical to square to very horizontal.
[*]You can now more easily select the ones that you want to use in the slideshow, and press F8 to run it.[/list]
Notes:
Regular folders can be sorted by Orientation as well, but it doesn't seem to work correctly (on my system anyway -- I think there's a bug in the sorting algorithm).
If you have multiple monitors, for instance a horizontal and a vertical one, you can pick the one to use in the Slideshow options (or use Alt-F8 to first configure, then run the slideshow).