I seriously doubt that even if Ran DID end up with Shinichi, it would last any longer than a year.
Genta and Ayumi are NOT for each other. I'm sorry. I just don't think so. xD Genta needs a woman who will whip his butt into shape. Ayumi is way too delicate for that job.
Genta and Ayumi... well Genta seems more interested in food than in detective work and is too rough and tough for her.
And besides, being all similar isn't a good thing. Ayumi and Conan still have differences like Ayumi being soft, kind, thoughtful, and sweet while he's mature, focused, irritable, and a tad bit sarcastic. Those are personalities that go well with each-other. He catches her when she falls and she's the person that makes him softer.