I’d be willing to buy several keynote templates if they were designed with accessibility in mind.
The biggest problem with the presentation templates we have at the moment is that the fonts are way too small. The second problem is that templates often contain way too much text (like several lines, multiple paragraphs)! Great presentations have as less text as possible. I know that I can delete the text they have, but then since the font is too small, I have to keep resizing font, resizing and moving text frames, sometimes even finding a font that looks better in a bigger size. We, who buy templates and themes, would like to have little to no effort at all 
We’re willing to pay for that facility.
I’d like something that was designed to be seen from a far distance. Most templates here can’t be read if someone’s sitting 5 meters from the data show projector.
Themes I’d be expecting to see and buy would be anything business related, specially pitch decks for startups, and scientific presentations, to demonstrate scientific projects.
Cheers!