Answer 1

The chance of a Gym Leader visiting is still unknown seen as Nitendo has revealed it.
The Rank of your Avenue has no effect on when a Gym Leader will appear neither will the amount of visiters or time your Avenue has been open. Ofcourse if you have had Join Avenue open for a while you have more chance.
Although, some people think that the chance of a Gym Leader appearing is the same chance of encountering a shiny.
Personally I disagree with this but...meh.

Comments

I agree too.  Its definitely not the chance of a shiny because ive gotten a gym leader 3 times in W2 but ive never found a shiny in that game other than haxorus and dratini

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I've had both Cheren and Draden come. If it's the same chance as a shiny pokemon appearing then why don't I have more shiny pokes :D
jk of course.

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No, not the same as shiny. Definatly.