I still think Xion being introduced isn't what ruined the game so much as that she was ALL The game had. And this relates back to the topic of this thread and yes, had Days came AFTER BBS I think it would've been better.
They could've showed us more about Braig instead of tiptoeing about him being this freaky person. "What does he know?" Would've been replaced with "oh shi- oh shi- oh shi-" at what he DOES know and what he's DOING about it.
Xemnas could've been shown in the Chamber of Repose, conversing with Aqua's armor or something without them fearing "spoiling" us about Xemnas's connection to Terra, and it would've given more value to him telling Roxas that 'sleep took you again'. Instead of one BBS-related comment that doesn't connect to anything solid beyond what we were not supposed to know already, it would've tied to what BBS told us and what Days showed. But it's like they expected us to know that Xemnas = Terra and go with it >_>;
Roxas and Xion both could've profitted from this, especially Roxas. Because then they could mess not only with Sora's Memories, but also Ven's, and then Roxas could've gotten his own identity issue instead of leeching off of Xion. And it would've been interesting - now that Xion took all the "Sora", the "Ven" in Roxas could've surfaced and he'd have been confused over Memories of someone other than the boy in red. But because we weren't supposed to be certain of what Roxas and Ven's bond was, Roxas was stuck running emblem errands while Xion looked like Ven as part of her being the game's true Sora.
Axel. Oh, Axel. Instant dilemma and conflict about his friendship with Roxas. Is here there for the orders? For Ven? Or for this new baby-person that appeared next to him? Don't leave US to think that, SHOW it, and thus prevent it from possibly DOWNPLAYING Axel and Roxas's friendship because hey, he's there for Ven or something.
Would also exlpain Saix's hostility better.
But showing us even half of those things BEFORE BBS came out would've been spoiling BBS, and still illogical since we'd need BBS to understand those stuff.
Still, I blame a lot more Days' wasted potential on it being between KH1 and KH2. Regardless of anything, they couldn't really DO anything because that might've contradicted KH2.