They would have to come at it from a different angle from Riven's, admittedly, but I personally would think it would still be workable. Working solely based on what we actually know of Demacia, military service is required, and a caste system exists, you can easily see someone of a lower caste who was always unhappy with the system, but unable to escape it due to her low standing. Then, due to, say, her unit being wiped out (probably something a little less on-the-nose as a mirror to Riven), she escapes her assigned role in society, and travels the world, learning the true value of personal freedom, before coming back to work on changing Demacia.
Demacia should be allowed to speak for itself, rather than having one person come along with the intent of shattering all our misconceptions.
I agree with this, in principle, but at this point the lore is completely focused on champion bios. Thus, any history or background we get on any city state is going to be, by necessity, filtered through the lens of that champion's perceptions. None of the city states can really speak for themselves except through their champions (which, come to think of it, wasn't really any different when the JoJ and Judgements were going), and I think that is an area of Demacia's culture that bears exploring, particularly since they've been so thorough in exploring the "evil" side of Noxus. Though, to be fair, with Noxus they've done a fairly good job showing the "good" side, also, if not quite as obviously.