I am not certain how the engine behind AO validates text/code, nor how easy said engine is to modify/hack to make it do your will; this may be completely ridiculous to consider, or it may be relatively simple.

I'd like to see the ability to use Acronym tag's. Here's an example:

<acronym title="Rest In Peace">R.I.P.</acronym>

The result is text that looks similar to the text R.I.P. hyperlinked, but instead of having a new destination load, it simply causes a tooltip to appear with the words Rest In Peace in the box. You can see an example of it here. Simply mouse over the word CAD in the body of the post.

I feel this would be beneficial in so far as it would allow people to identify Acronym's for others easily. I know some site management plugins will automatically format acronyms as well, from a predefined list (that may be way too much for AO at this time.)