As of right now there are only two options to highlight documents. One is freeform (the circle cursor that allows you to draw out a highlight) and the other is text selection highlighting. These are great , however, Adobe Acrobat DC decides which one you can use and will not let you switch. Also there are times when OCR cannot recognize the text on a printed/scanned image so when I go to highlight it highlights the random whole sections of the page. The work around to this is using the rectangle tool but now that tool is limited to being a highlighter and is a hassle to switch back and forth when needed for something else.

What I am proposing is that with the highlighter tool selected you should be able to hold down a key (ctrl/alt/etc.) that would then bring up the freeform highlighter regardless of text recognition.