Now in fact there are still reasons we need key codes that are different to the eventual text representation, e.g. for cursor movement and other special characters like function keys.
Looking at the Xorg source code now, there's a relatively fixed notion of what a keyboard can actually do. I suspect that conceptually a somewhat backwards-compatible extension would be to have a new dedicated kind of device, that is exposed (similarly to a keyboard with an integrated touchpad) separately to the other functionality of the device.
In particular, I'd like to keep the keyboard in "regular" mode as long as the host hasn't signaled that it wants to use the extended functionality via, presumably some part of the USB negotiation. Only at that point would the extension be activated and the keyboard output would be sent via it. The regular keyboard device would then be virtually unplugged.
I suspect that this is better than having two devices, one for key codes and one for text input, especially because we'd not be able to guarantee in which order the two devices would be read. This is less / not a problem between a keyboard and a pointer device of course.
Now given that X11 isn't the interface most applications are written against, how would the text actually arrive at an application? I'd imagine basically extending the whole event handling by one more event, the text event, which wouldn't correspond to any key (thus, it can't be in a pressed or unpressed state). In terms of GTK+ and QT this might be even easier than for a lower level application since many applications will only want to deal with text input and pre-defined keyboard shortcuts anyway.
Speaking of which, what does "Ctrl-C" actually mean? Of course the mnemonic for "copy" is in there, but also the "control" modifier. How well does this play with text input? Not at all, and I believe modifiers work better logically on the key code level; for text input I imagine other modifiers like "emphasis", or, more specifically, "bold", would make more sense, possible "URL", or "footnote".
Overall there can of course be modifiers active while text input occurs, it's more a question of whether they (can) be assigned any meaning without falling back to the flawed character equals key press comparison.
What does this gain us? Ideally every application (or more accurately: each toolkit) could now drop logic specifically to translate key codes to text, since all of it would already be handled by the keyboard itself. Keypresses would come in via the same interface and be used for specific, non-text functionality.