Why in the hell should there even be a boundary? One of my friends played Warwick mid and completely destroyed his lane opponent, carrying the team. Another time, I played AP Nasus Mid, and did reasonably well in lane phase (Hint to anyone who wants to try this: he is incredibly squishy if you don't build him tank-after lane phase it didn't go to well for me). There shouldn't be "Top Champions" or "Mid Champions", there should just be Champions. Yes there can be places where they work better than others, but that doesn't mean they have to be forever pigeonholed into those spots.
It always bothers me when people complain about someone playing ranged in top, about how they "don't belong there, because they completely reck other top laners" - not to be rude to those people, but if you think it's so unfair, then just play ranged as well! You may not like it, but if you're only reason for not liking it is because you keep getting beat, and you think that the only thing that can be in top lane are big melee tanks, you have no leg to stand on.
Champions should not be designed for lane positions, they should be designed for roles on the team. If you want to play a 5-man, all tank, all melee, 2-1-2 lane arrangement, then by all means do it! It may suck, it may dominate, but you played the game! To hell with what pros play, there are plenty of other ways that can win a game. Look up any youtube video about team comps like all ADC, a heal squad, or even all knockups; granted they will probably put the best footage in their videos, but they still played the game, and if something works even once, it can be justified to work again.
Sorry for the rant, but it's just something that really bothers me.