Depending on what works best there might need to be 2-4 topics, each covering one league of its own or two etc in each of the league sub forums.
Then you should start a few in each league right away! I love seeing arena decks and be inspired by creative ideas!
Btw what's your definition of good? Will top 50 rank do?
Gold and Platinum is pretty similar and as i mentioned before the age requirement is simply there to stem the tide of decks.Top 30 might be more prudent and remember that the arena is still young. ALOT of decks can get into top 50 in a couple of days. The ability to stay is what separates the good from the rest in gold and platinum.
I have no idea how fast bronze and silver moves. You have to wage the amount of possible decks with teh amount of users who can submit and use the forum. Is it even feasible to have one for bronze? Bronze probably should be limited to top 10 (wild guess) and only win/loss ratio.
Silver no guess at all.
On a side note
Good is different for each league. I would prefer if someone who feels they have a grip on bronze and silver made those topics.
(Does a single deck in bronze have a more wins than losses? Considering people can attack it with fast rushes that wins more than they lose theoretically the answer is "No none have positive stats in the end", while another thought says that all the mediocre and casual players with no idea of the forum and deck theory knowledge fights the bronze arena could lose in staggering amounts to the better decks. I don't know, I don't do bronze and as such someone else have ot come up with a good solution to how to handle bronze.)