I don't know that there is much difference between the two. In his comment Hank suggests that "Maybe you will ..." can be a kind of invitation, but I think this depends more on context than anything else.
London is a beautiful city. Maybe you will visit it sometime.
This is just a recommendation, not really any different from "you should visit London".
The difference between this and your example is the direction of the verb, to come to London. Because I'm moving you toward me, it implies that I live in London, and I'm inviting you to visit. It's really no different from saying "You should come to London" -- again, an invitation.
But not necessarily a formal invitation. As Hank mentions, if it is phrased as a question it more strongly implies invitation, but otherwise you really should clarify and confirm before making the trip. Of course I know some people who would go anyway and just "show up on someone's doorstep", claiming "Hey you said I should come sometime!" But that's more about personality and culture than about English.