Ok, how about a custom "Honor" system for D3? Something like community-made challenges or meta-games that can be chosen by the player to be tracked and maybe even shown on the character in someway. Well I know this thing is already in place, but playing Diablo has a strong aspect of boasting about your characters and stuff.
Lets make the idea a little more clear: I am talking about a modding tool, in which you can define an "Honor", which then can be chosen by each player to become active for each of their characters. The Honor can then act as sort of an achievement, i.e. stuff like "Kill Diablo on Inferno running naked" or some sort of comparable score, i.e. stuff like "Longest elite killing spree in one run" or "Fastest Ghom kill". The Honors then shall be displayed on the character in some fashion, most possibly being displayed on the character's link. Note that the Honor system will not alter the normal gameplay by any means, i.e. there is nothing that you can do with it in place that you cant without it, I mean besides the ability to "Honor" your various "Deeds" in the game and boast about it publicly.
So what will this actually do for the game's health? Well for one thing it seems to me that already these kinds of meta-games are playing a serious role in the game's longevity. Various challenges are offered in the community with an increasing rate, stuff like the self-found only or the random hardcore challenges. These kind of meta-games are actually being supported by developers as well, because they help players enjoy the already in-place content rather than requiring additional (most possibly periodic) content updates, which can be really costly specifically for a non-subscription-based game. However, without any actual reward, many players do not find these meta-games attractive enough.
Such system would not just give players enough reason to try such challenges, it would effectively offer playerbase many systems they have been requesting, ranging from the aforementioned self-found title, up to ladders and even community organised PvP tournaments.
But is this thing like hard to implement? Not really. Basically it requires a simple scripting language for describing Honors, some additional callbacks in various in-game events (monster killed, skill used, etc.) and a little change to the player data the game tracks (I dont know specifically how this thing is coded, but my guess is an "enum" on each item would suffice). Yes I know that this will be a server-side thing and not letting malfunctioning "Honors" interfere with playing experience and doing so efficiently can be a little though, but hey, it will be far easier than implementing ladders, tournament support, self-found recognition, etc, etc separately, and also, this is like a wishing and dreaming post, so forget about technicality, what do you think? would you enjoy such thing, or consider it a waste of time, or actually find it damaging the game?