I would like to have an iPad replace my desktop, but I think the 12.9 inch iPad is still too small. Any chance they would ever come out with a 17 or 20 inch iPad?
It's hard to say "it'll never happen," but I think anything much larger than 15 or 17 inches is not very likely for a mass market, touchscreen, portable device - the ergonomics start to get difficult - holding such a thing, for example - the weight and bulk are problematic, as is arm movement.
The larger the touchscreen, the larger the arm movements needed to use it. That's what 'gorilla arm' means. Unlike mouse, trackpad, or trackballs that can be scaled (small movement of mouse = large movement on a large display), touchscreen is 1:1.
I don't think there's a large market segment willing to deal with the ergonomics issues. That doesn't mean there won't be some users who will appreciate having that size to work with, especially pencil/stylus users who want to draw/manipulate a large canvas (graphic arts, CAD), or users of virtual work surfaces (audio mixing boards, aircraft instrument panels, etc.). Chances are, it'd be a very big-budget item - Mac Pro price range, not iPad Pro pricing. There could very well be an optional, Apple-designed support cradle for displays that might range as large as 40 inches. At those sizes and for that kind of usage, it's a good question as to whether they'd even be battery-operated.