If anyone's going to become Reimu, it should be Iku.
Of the mages, Alice has dolls, Rumia is perfect for a dark knight, Marisa just plain murders everything, and Patchy is basically Marisa with even more power but less speed. Iku's main selling point is tanking magic, which doesn't sound bad, but it's a lot simpler to take care of enemy mages with cold steel, considering magic units are usually low-HP low-def unless they're endgame bosses. And by that time, upgraded Byakuren will have light magic and won't be too bad at taking magic hits either. So, yeah, Iku seems pretty expendable to me.
But if we're going to convert another unit into the lord, I think that it should probably be somebody that's close to average anyway. Unfortunately, there's only one character in our lineup that fits the bill, and I personally don't want to be the lord. So I'd much rather someone take Reimu, but if that falls through, then it is inevitable, and we are throwing Yukari out of the Hakurei Shrine permanently.