I think a beneficial change that could possibly help a lot with usability is tagging. Forums by design tend to have pretty clear-cut boundaries for a content's location. However, when you're on a specialty forum with an overall topic being the only thing discussed, I think that it becomes a little limiting to have so many divisions when trying to explore for content.
For example, if we take something relatively simple and clearly defined, such as the 'member tanks' section, you will see that is is divided into regular/large/monster/nano/AIO/cube etc. depending on the size and dimensions of the aquarium in question. This kind of division seems a little obtuse to me because it's pretty much user defined and not very intuitive for navigation purposes. Tons of "large" and above, AIO, and nano tanks are in the regular section (which obviously has more visibility than the smaller subsections). Someone who WANTS to look at "monster" tanks will obviously click that section, but in doing so, tons of great builds are hidden because the average person isn't going to have interest in searching out what a 700 gallon reef looks like.
The tagging system is really awesome, but it's not utilized to its maximum potential because frankly most people don't know what it is or how best to utilize it. It's like how we have a very popular and active "Show off your Red Sea" thread because there was a genuine desire for people to be able to locate and share that content. However, if tagging was used, 'Red Sea' could be used by members when creating their build. Letting other members add additional tags could help this by allowing the site population to help index helpful content without relying on a somewhat clunky search feature or relying on thread creators to do it themselves.
By allowing/adding this feature you're basically making it much easier to find content without relying on the search tool or digging through tons of different subsections on the forum to find content you don't know exists. I don't know how feasible it is, but I would love to be able to go into an area like the member tanks and click tags to quickly narrow down build threads, such as the tank dimensions, aquascape, type of rock used, equipment choices, glass or acrylic, led or t5, closed loop, sand or barebottom, etc.
Even if you don't agree that all people should be able to add tags to whatever thread, I'm positive that some users would gladly wear a nifty badge and go around adding them as needed. It's really an incredible feature that should be explored more and/or made more visible to users.