I am not a game designer... but I know where some live.

I understand the desire to know what changes will be coming in future patches and expansions of WoW.  I read the blue posts, the PR releases, convention coverage, and the info from data mining.  I read the blogs of people who suggest changes to blizzard.  I've even made my own suggestions on fine tuning my own class.  I've spent a short amount of time on the Public Test Realms to see upcoming features.

I am not a game designer.  This job requires experience and skills which are not gained from simply playing a game.  It's not that players, especially those who spend a lot of time dissecting the game, wouldn't be good designers, but the skill to play and understand the mechanics is different then those of the game developers.  If you are going to make a suggestion to the Blizzard designers then make it a simple and focused suggestion.

Create a simple description of your idea.  If this takes more then a sentence or two, then it's probably needs to broken down into multiple smaller suggestions.

• Add a new Guild Bank tab option that works like a guild member only Auction House.

Identify any impact on existing game features.

• Guild Bank access for the tab:  Withdraw only = buyer only, Deposit only = seller only, Full Access = buyer and seller. 
• Limit the number of listing and purchases per member.
• UI interface changes???? I'm not qualified to suggest or understand the implications... Ugh.

Identify any problems which the suggestion would need to consider.

• Tab is just used as a very large regular tab by listing items with very small buy out prices.

Identify possible solutions to the problems.

• Limit the number of listings.  
• Charge a minimum fee for each listing regardless of buyout price.  Minimum or percentage which ever is higher.

Identify edge and test cases:

• Standard AH test cases (I'm sure blizzard testers have a test plan for this already)
• The guild master disbands the guild.
• A lister leaves the guild.
• So many test cases and I'm not being paid to think them all up!

This is a simple example and I don't even feel like it's complete.  I am sure there are more problems which could be identified.  I am sure there are more test cases which should be considered.