The ability to play chess. >P
As for multi-classing, technically, we already have it with augments... which is an iffy mechanic for me. I don't hate multi-classing, but I don't really like it either. >P You should make the classes -work- without having the player feel like they need to multi-class to offset the weakness of their preferred class/path of roleplay.
Uhm... if KotL were actually PvP balanced(which it's not), PvP world-type objectives -can- be fun. In the case of Achaea, this constitutes two forms right now. Icon wars(lame) and city-raiding(not -as- lame as icon wars, but can be lame because there's no end-goal objective to say, "okay, you've won. Stop doing it for a while).
In the form of Guild Wars, you had the Faction battles in Cantha. Where Guilds would align with either the Luxons or the Kurzicks and battle on PvP maps(3 groups of 4) where the outcome of these many battles fought at the same time in seperate instances and can move the border of an in-game map and give some small benefits to Alliances(groups of guilds) at the top of the alliance ladder. Considering Guild Wars is pretty balanced, especially in PvP, it was -really fun-. Also, the lack of long-term penalties for death(Guild Wars only has a temporary death penalty that causes you to lose 1-60% of your HP and Energy until you return to a town--which occurs at the end of a PvP instance, usually).
Achaea also has wilderness maps, which make the world -bigger-... which can be bad(I mean, all sorts of area to cover can be irritating for non-mages), but... they can be nifty. And since you don't need room descs(since you'd get an ASCII map for the "desc"), it lets you make a larger overworld that... yaknow, has geographical sense.
That's about all I have, I guess.