Here's my opinion on Ai and Conan.
I prefer their pairings from Ran and Shinichi. I just find the idea of the detective and the girl who betrayed the evil organization much more interesting than Ran and Shinichi, who knew each other since childhood and developed their feelings over the time and who were in love with each other since vol 1. I still like Shinichi and Ran, but I find Ai and Conan chemistry much better and like I said they are more interesting. I'm not delude that Ai and Conan will end up together. We saw Ai having some sort of feelings to Conan, I personally believe that she has fallen in love with him unlike Conan. He cares about her very much and he would always protect her (I love when he saved her from the bus and ran after her, when he thought she was going to run and more), but he doesn't have any romantic feelings to her.
I was wondering lately, what if Gosho would make Conan to develop romantic feelings to Ai between vol 18 and vol 42. Would Conan choose her over Ran? I guess it would depends, whom he would love more and he probably would choose Ran anyway.
I still like the moments between Conan/Shinichi and Ran, but IMO they don't have this spark/chemistry which Conan and Ai has.
Of course, Ai will still have a happy ending, she must have. Until now, I didn't mind her staying as a child, but after some thinking, I came to the conclusion that staying as Ai would mean running away from who she really is and I think, she can still be happy as Shiho. She could work as a scientist, in the police/fbi, as fashion designer or be even Shinichi's partner in the crime investigation. There are lots of opportunities for Shiho to enjoy her life.
I wonder if Ai/Shiho would end up with someone at the end of manga or have love interest, at least. I personally don't like the idea of
I also think that Amuro could be good match to her. I know that they had very little interactions but since he
, they could be together. I know there are low chances for them to be together but I would prefer them more than Shiho with
If Gosho would decide that Ai will stay as child and he paired her with Mitsuhiko, I won't be completely happy with this. Like I said, Ai still can be happy as Shiho and she doesn't need to relive her childhood and honestly, I don't really think if it can count as childhood, since she's not a child, in reality. As for the Ai and Mitsuhiko, I don't mind them, though I still prefer much more Ai/Shiho and Conan/Shinichi, even Shiho and Amuro would better than them. I just hope that we won't get any hint that Ai likes Mitsuhiko in the romantic way, out of blue, this would be horrible.
I won't hate the ending, where Ai will stay as a child, even though I don't agree with it. However, it would still be a nice ending for Ai and I believe she would be happy as Ai, but like I said, I beleive she should return to being Shiho.