If we're still theorycrafting about making an MMORPG sandbox without instances, then there has to be as much space as possible. Housing space was problematic in UO, it wont be better now that the genre has gone mainstream. One nice thing about WoW were all the little caves in pretty much every zone. In the WoW game model they became boring predictable but in a more open world they could serve as xp spots for a singe group or a duo (in WoW you quickly learned there are only like 2 map layouts and you dont need to go in unless you have the quest thats surely around to point you there).
I think you focus too much on the sandbox and neglect the RPG part, too. Let each character become to sum of his journey and not just the last 2 months, like newer item treadmill games do. The +gooder eqipment during the leveling process is utterly pointless as is, any stats on those items are either negligible, replaced within the hour or on a dozen nearly identical items within the same content range. Nobody cares about them.
On the other hand if you only give a class the basic abilities easily (assuming you go with set classes and not freeform like UO/EVE). Have only few skills in training academies in the cities and make players *play* the game for the other skills. Gain the trust of the old wizard to be granted a scroll to learn his unique version of fireball, seek out the grandmaster monk to learn his special technique, find the necronomicon and learn that rare spell, help the town guard against the invasion to train in their ranks for new abilities, faction up with the remote elf village to learn under their elders, etc etc, Endless possibilities, really. You can have slightly different versions of the base skills as long as the power level is similar, as well as epic spells you have to really work for but that are worth it. As long as the game goes with a limited available actions model like GW1/2 or D3, the characters dont gain more pure power, they just gain more options. Kinda like learning more skills in EVE allows you to fly more ships, slot more equipment etc, but you cannot bring all that to bear at once. Another angle is that you can have class skills, archetype skills and even skills everyone can master. Ideally I'd like to see the EVE skill model, time based, but learning the "skill book" through game play instead of just getting it off the market.
That way old content also stays interesting to newer players instead of skipping past it asap, and for old players to go back for a different version of fireball eventually because they want to catch them all, or changed their preferences or whatever. nobody goes back to old content for stats (people do for skins though, a second angle to keep old content interesting). Throw in a low power curve or GW2 style level-downscaling (or a system without levels) and your growing world stays much more relevant then in the WoW/EQ/clones model.