Now, I don't want to pile on with all the negativity, but there's some important context here that you should be aware of.
There have been many,
many, MANY attempts to write Pseudo-routes. There's some disagreement on what exactly constitutes a pseudo route, but, to date, there are precisely TWO completed pseudo-routes in which everyone considers them indisputably pseudo-routes (i.e., Hisao x girl, same general time frame, multi-act structure, etc.) (For those of you keeping score at home, that would be the
Rika Pseudo-Route by Hearts/Rikabro and the
PseuPseuSu by Lunch/Scissorlips). In both cases, things became very difficult near the end, and the authors both more or less suffered complete burnout shortly after finishing. Most other attempts have been prematurely shoehorned into a rushed end (with the author admitting that he had pretty much given up), have been explicitly abandoned, or, as is the case with yours truly, continue plodding along at a glacial pace. This is so endemic that we have the notorious "Miki Curse," relating to the fact that so many Miki pseudo-routes have been attempted, but they all seem to die catastrophic deaths.
So we're all a little bitter, jaded, and heartbroken. It's like the (historically inaccurate) movie
Anastasia. We are the dowager empress, and you're Anya. We've encountered countless people claiming to be the true heiress, and they've all disappointed us. So we've decided that enough is enough. You may very well be the true heiress, but you have a long hard road ahead in proving it to us. And it's likely going to take more than a good-looking lovable rogue and a memory of peppermint oil to convince us otherwise. We're
tired.
And, to be honest, there's some degree of pattern recognition here. The attitudes of "I want to do something big" and "this is something to keep me occupied while I work on a bigger project" and "[completely 'original' idea]" tend to be warning signs. They could mean nothing, or they could mean everything.
And, to speak from personal experience (and from talking to others who have attempted it), writing a pseudo-route (or any long-form work) is
hard. Writing one tends to take at least a year, often more. Even if you have a storyboard, outline, and all the beats planned out, there's still quite a bit of work in simply adding the connective tissue, to say nothing of the constant cross-referencing to make sure it's both consistent with the original and with itself. And a lot can happen in a year. Off the top of my head, since starting my Misha route, I've changed jobs, had a kid, and been diagnosed with a disability myself. Others have had events like deaths in the family, nervous breakdowns, or simply "not feeling it." It's a significant commitment, and not one which should be taken lightly.
Finally, on the subject of originality, I'd be inclined to somewhat disagree with Oddball. Originality is important, but it's far more important that something be good than original. And a key part of that is determining whether your original idea is original because no one's thought of it, or because no one's attempted it. And if no one's attempted it, why haven't they? There are plenty of ideas that haven't been written because, frankly, they're bad ideas.
Now, is your idea bad? Let's look at what we have here:
Not Hisao - Fair enough. There's plenty of Hisao-focused stuff, and plenty of good non-Hisao stuff. Just pointing out, though - a lot of people won't consider it a pseudo-route unless it's Hisao x ?.
Iwanako - Not a bad idea per se, but definitely a difficult one. If she's not going to Yamaku for some reason, it's extremely hard to justify at all. If she does go to Yamaku, that comes with its own set of issues.
Borderline Personality Disorder - Nope. Canon has Yamaku specifically not designed to cater to mental disorders (because those require and entirely different set of facilities, treatments, accommodations, etc.). You'd be better off having her end up in some kind of freak accident that leaves her physically crippled, but that leaves the question of "Why Iwanako?" Additionally, she'd very likely be aware that Yamaku is the school that Hisao went to. There are actually a significant number of schools like Yamaku in Japan; she might want to avoid the one Hisao's at (especially if she went Borderline over the whole incident, but that's another story entirely).
You want Iwanako at Yamaku? Best bet (which hasn't really been explored) would be her coming to visit over the summer break post bad-end (Rin's and Hanako's are most conducive to this setup). Summer breaks aren't necessarily at the same time, so she could be visiting while Yamaku classes are in session, for that matter. That gives plenty of opportunity for Iwanako to interact with the entire cast, if you so desire.
Whatever you choose, I wish you the best. And just remember, we're not discouraging you from your current plan because we're mean people. We're discouraging you because we're mean people who have legitimate reasons to expect that your plan is a bad idea.