While I might not be an expert on manga, or anything of the like, i would still like to put some input into this conversation.
The biggest clue to who I think Soma will end up with is hinted with what Joichiro said at the beginning, which is that he needs to cook for someone he (Soma) loves.
First off, I believe that an Erina and Soma romance is one of the three logical conclusions. I came up with this being one because there are a few factors that point to it. First, there is Soma's declaration that he will get Erina to say that his food is delicious, which fits in with Soma's father's advice. There is also the fact that Erina usually acts flustered around Soma, and also the fact that he somewhat broke her panic when her father came to her booth during the Moon Banquet Festival. There possibly might be a "knight saving the damsel in distress" situation coming up, and that generally leads to a deeper relationship. I also believe that both are helping each other grow, both as people and as chefs. That's why I believe that the "bratty" girl personality was shown for she could be rewarded for changing, not because she stayed the same through out the series like some static character.
However, as I said, there seemed to be three logical relationship conclusions, I believe the second one is Megumi. While she might not be as flustered around Soma, Megumi has definitely shown a lot of growth as being a chef and a person so far because of Soma. Megumi has also worked with Soma a lot, and has tried many of his recipes, even though many were bad, fitting with Joichiro's advice. They also live in the same dorm, which might mean something.
The third logical relationship with Soma is with Mayumi. While there is not much about her so far in the manga or anime (if it continues), there are a two reasons why this is still one of the logical conclusions. First, she is a childhood friend, which means that they have technically spent the most time together, and they have shared many memories together. There is also what Soma said to her near the end of the Market arc, where Soma tells Mayumi that he will be willing to whip up something for her whenever is in the district, which seems to be another tie-into Joichiro's tip.
These relationships seem the most likely to me. However, there might be information that I missed or hasn't come out yet that points to another relationship.
One thing I definitely think will happen is that at least two of any possible girls that he will have a relationship with will go into a shokugeki against each other to decide who will have him, even if he doesn't agree with it, because it is "final."
Remember, this is just an opinion and theory. You can critique it, but I ask you to do so politely. Also, I think another good question, which might be elsewhere, is why Joichiro never graduated from Tootsuki.
