Half-Ramp Harry... now that'd be something terrifying indeed.
So the WAG would be Bianca half-turns Harry, and ... I dunno, maybe Thomas/Michael rescue him before he fully-turns?
So Mirror!Harry gets some o' that St. Giles training (that uber-practical "hard choices must be made" St.Giles training)? From a theorycrafting perspective, being emotionally out-of-control is bad for both wizards and half-Ramps; more than once, Harry has "expended" his emotion on magic. Could he do the same and get Ramp-urges under control? Go from all-tattoos to none, via one emotion-fueled spell?
Maybe learning-to-Ramp alongside Susan?
I still think a living Susan would be all kinds of fuck-with-his-mind & torment for Harry-Prime, so I expect Jim will do that thing (and several other f-w-h-m torments, of course).
I like this theory! This moment -- the Bianca showdown -- is actually my #2 pick for Harry's choice/decision point, though I can point to a bunch more maybe-this/maybe-that choices.
I still like that latter on-the-beach-with-Susan moment even better, because it's all about love (and loss), with no violence or combat clouding the issue; also, it's the very-very end of the book. As we increasingly see as the series develops, "Love" is a critical element of the Dresdenverse, possibly THE critical element (possibly alongside Free Will).
So, a choice made strictly and purely around the issue of "Love" has enough metaphysical weight to create a whole new parallel...