I liked XIII too. And I'm not against Lightning being in XIV, either. I know, as well as others, that her quest MOST LIKELY won't affect the main scenario. Still though, it DOES take from the uniqueness of XIII and XIV respectively. Even XIII and XV (versus XIII), who share the main world lore, will have nothing to do with each other apart from the same mythos regarding L'cie and fal'cie and all that.

I do think, that the people who are bothered by it simply need not do the quest, but OP's plea is more along the lines of trying to prevent the floodgates from opening. Like.. "Hay, we did this with Lightning, lets do it with Sephiroth and Cloud and Tidus and all the other awesome FF series characters!" Which is VERY different than the Cids and Biggs & Wedges we've had pop in the series.