It'd be really handy if one of the #show options for macros was the stack of buffs/debuffs on a unit. So for example:
#show [target=target] Deadly Poison VII
would show show the stack size of Deadly Poison on your target (and the cooldown spinner thing for time left on the stack). Clicking the macro button would cast Envenom. This is definitely doable through action bar addons, but adding it to the macro code would be nifty.
I looked for a suggestions thread on the WoW UI forums but didn't see one. If there's a better place to put this suggestion where it's more likely to be seen by Blizz, please let me know.