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.