There are some excellent ideas here and I'd love to see more. Here is a quick list of five I'd like to see:
1) Overrep Bonus
Several people have mentioned a problem with the current state of Isle of Thunder dailies, namely, that players hit Exalted and stopped. This causes overall participation to drop, and slow the rate of Stage expansion. While this is a harsh example, there are other times people would want to continue doing dailies that have a rep attached, even though they are maxxed out. Think Sky Shards, for example. Or continuing to run a dungeon in BC for that Raven Never-Drop Mount over and over.
I would not mind seeing some kind of bonus for gaining rep in a faction you have at maximum. Maybe when you hit +1000 over exalted, you create a +250 rep BoA item for that same faction, and drop back to Exalted +0. Or a small amount of experience, JP or Honor depending on the faction/level. Or maybe a bag of spoils. Just a little something to make the "overrep" useful.
2) Stupid Mistake of the Tiger
I'd like Blizzard to go through their random item stat generators and remove every single useless combo from them. I admit, it probably would have very little effect on gameplay for most of us. But finding +agility plate, items which have no useful value to anyone in the game, this many years after the stats were changed, is crap. Blizzard should be ashamed for having such things, even if they are such a minor part. And hey, we've ALL worn greens at some point, haven't we?
I have personally sold Ravager Bands of the Wolf. Agility/Spirit. Nobody uses these!
3) Otra Lengua
I would like some way for characters to learn another racial language. Horde/Alliance communication will probably not happen, but a tauren who learns to speak troll makes sense. Or a DK that speaks Undercommon.
4) Deja Voodoo
I would like the ability to re-play a quest I've done before. Maybe with an ilvl reduction, as some have noted before? Maybe with the ability to purchase a different item at the end for a JP fee (500 green, 1500 blue, 3750 epic?) There are some quest/quest chains I would not mind re-doing without leveling another alt.
5) Chairman of the Board
I want a damn sandwich.
Every innkeeper from here to West Dwarven Jabip sells meat, bread, and cheese. Where are the sandwiches? I can see myself eating a Roast Beef and Fine Aged Cheddar on Mag'har Grainbread. Or a Rhino Dog with Cabbage Kim Chi (because there's no saurkraut) on a Buttery Wheat Roll, no mustard. Or Thick-Cut Bacon and more Thick-Cut Bacon on...well,anything. Food's already divided into categories because of hunter pets. We know where the bread is. This is doable!
Rule 1: Must acquire a sandwich board through N+1 Epicurean Awards.
Rule 2: Using the board brings up a UI with 3 spots for ingredients. The first must be bread, the other two anything which aren't drinks.
Rule 3: No conjured items.
Rule 4: The created sandwich gives the same health and mana as the highest of the 3 ingredients. No stats are given.
Rule 5: The created sandwich <Describes the Ingredients> in the same way as the crafter of an item.
Rule 6: Eat it. Sandwiches are awesome.