For example, if you've typed enough to narrow it down to one candidate, it shows up on the same line as what you're typing, instead of the line below it like it would if there were at least two candidates. Changing the 5th element of ido-decorations to show up on the next line isn't a legitimate solution because sometimes that's used to visually wrap around a partial match, which ends up looking very confusing and strange on the next line.

How hard would it be to change/edit/hack ido-mode to work more naturally when showing results vertically, maybe as an official option of ido-mode, or a fork?

I've tried helm, and it's way too overkill for this small task. It's much too heavyweight, uses buffers and leaves them sitting around, and does way too many things. Now ido-mode is *almost* perfect for completion and narrowing when used vertically. It just has a few quirks because it was never imagined to be used this way by the developers.

-Steven