There are significant differences between a full sandbox or 'sandpark' game, & a sandboxy themepark - WoW would benefit enormously from becoming a lot more of the latter than the near-full themepark it is now, but ironically enough, would not benefit as much from trying to be an actual sandbox or nearly such. In fact the latter wouldn't even be feasible, from a technical, financial, even legal (shareholders etc) point of view - this is exactly what happened with Star Wars Galaxies several years ago, what they call the 'Pre-CU disaster' (in reverse, of course - they had basically a sandbox design, & because WoW's themepark got SO explosively successful in 2004-2005, recklessly turned their game around to be a cheapened themepark). If they did that with WoW, turn a full themepark and its millions of subscribers relatively content with the current state of affairs, into a complete sandbox, that'd be just as irresponsible and backstabby as the SW:G Pre-CU failed moneygrubbing gambit, no matter how gloriously more dynamic & engaging the non-static gameplay naturally emergent from sandbox content is.
See that's what I mean when I emphasize there are always ways to make emergent dynamic gameplay adjustments to a themepark design while not damaging/removing what is already existing, so fewer people complain, you didn't throw the baby w/ the bathwater to implement the sandboxy features. This touches upon what Ravenmoon said about 'raising an empire' etc, that type of play is absolutely engaging riotous fun and yet, just has a really hard time being realistically, holistically integrated into the aged systems of an old classic themepark like WoW. This may understandably seem terse of me to say or contradictory to what I'd always been advocating, but it's important because throughout the last decade in the reign of the themepark design in MMO space, time & again some very well-intentioned people have suggested that devs implement such radical sandbox features out & out, and it is unfortunately those outlandish suggestions that have emboldened dev teams to lump all dynamic, undirected, emergent content design into the same "unrealistic, out of touch with the 95% rest of themeparks' Halo/CoD-type playerbase" basket, thus leaving no room for more moderate steps in emergent sandbox gameplay to have any chance at the development table.
Actual empire-building & permanent or even mid-term temporary political power shifts/NPC disappearances just doesn't mesh with what the devs & executives have had in mind for the core design of this game, for a long time. So you have to find ways to implement some fun stuff to do, or different ways to do existing stuff, that won't clash, but to the contrary, that will harmoniously meshwith the existing core vision for how the game's themepark systems have always functioned.
For example, one of the ways I envisage dealing with the "..So I'm finally max-level.. what now? I mean, I was having so much stimulation, so much fun every level I was gaining, always a new spell/talent/glyph here, new items/power/feature there, and now, I'm just resigned to a much slower, repetitive gear treadmill till I gouge these eyes out to static boredom..?? =/" is the following :
~> XP costs when dying, but only under certain circumstances/in certain areas.
~> XP costs to do really engaging, designed-to-be-long-cooldowns things in the game, but mostly things that are cosmetic or affect world zones, not raid readiness or pvp advantage in any fashion or else it starts becoming a 'requirement', and thus giving an unfair, unreasonable incentive to play 24/7 in order to have as much recurring XP as possible to spend on say, more powerful flasks for a raid or a temporary speed boost in a BG.
~> An even more radical, sandboxy way to handle them would be the capacity to spend XP points, or HP, on the fly, in addition to the regular mana/class resource pool costs, when casting a spell in a world zone (so it leaves endgame PvP/PvE unaffected), to have a more spectacular (mechanical & cosmetic), on-cooldown (to prevent abuse) effect.
This would allow people a balanced, non-endgame-disrupting reason to do stuff that gives them XP even though they're already max-level (losing XP would never de-level you, that's cool-sounding but against Blizz core design..I know), perhaps even by having to use an item to give the buff that enables the acquisition of such post-max XP points, an item that only works in world zones, thus sending more players out there instead of staying in capitals to wait for their queue & troll tradechat 24/7.