I played this game nonstop for the first month, and in the last 2 weeks I've played once. It's impossible to ignore the frustration of the community over lack of "endgame". The first two ideas came from high profile players (Kripp and Athene).
First, selling only gold in the RMAH does a lot to for players not willing to turn a game into a real life monetary struggle. Players that wish to spend real money will still buy gold, then turn that into items, as their money is worth less than their time. It's a slight inconvenience to buy gold first then spend gold, but at least ALL the items are in the same AH and that will increase your chance to quickly find the best weapon available. This will force the real money prices up slightly since it will inflated with the gold, more so than the current system where real money holds value insanely better than gold. Think about it, as botters sell gold cheaper and cheaper, buying items with legit gold will become less and less viable, people will realize the majority of the better gear (in a 150% gear based game) will be pay to win (either by buying gold or using RMAH), this turns a lot of "gamers" off. This is good for blizzard mainly because as gold inflates through the roof (as intended I'm sure), people buying gold wont be capped at 250$ per item.
Second, there is a need for end game. A ladder system would increase playability after 60. Hopefully 2, a pvp ladder and a normal ladder. Think about it, this game shipped with no ladders, a ghost town social interface, and no pvp, I can't imagine anybody at blizzard didn't see the mass quitting/QQing situation that were in right now. Currently the only endgame is NV, which is sad. You hit Lv60, and just for example say you bought the best gear in RMAH, what good is NV, what is there to do in the game?
One thing I can think of would be another mode after inferno is beaten which would be either small NEW dungeon runs or NEW boss runs based off existing or new content. It would be ideal if you had to earn each of the new dungeons by beating the previous or having another leveling system for unlocking new missions.( Think CoD special ops) The new dungeons could even be made with terrain/mobs in the game currently just using some creativity. Imagine if the community could make these dungeons, then sell it on RMAH? Look what the map editor did for wc3? Things like environmental variables traps, usefull terrain, and items that only drop in said dungeon runs would increase the viability and give the player something to work towards after Lv 60. To get to inferno you have to basically beat the story 4 times, after a few chars you really don't wanna see the same story anymore. Cleverly made dungeons, especially ones with new terrain/mobs would increase the replay value. Having some kind of stats for the new mode would help too. You could even do some in a survival sense, sort of like jar of souls, except the mobs never stop. It would be like CoD zombies in a way, an unbeatable mission, where people could be ranked based off how long they lasted in a never ending mission. Can you imagine the replay value for an unbeatable dungeon where you were ranked based off time survived, I sure can.
Another idea for a post inferno mode would be a mode where you had a % to lose gear on death, almost a medium core mode. You could offset it by increasing the MF on NV or letting it stack 10+ times for 1 hour or just a constant MF buff that would increase as you progressed.
After inferno you could make a naked mode difficulty, where mobs dmg/hp would be based off being naked of gear. This would interesting to see how people stack up without having a huge gear edge, which in reality makes the non pvp game less fun. You can watch Athene killing every boss without looking at the screen, how is that rewarding, how does that require any skill? What fun is facerolling through mobs? The game is way too item based (touched on by Kripp) buffing hero stats and nerfing weapons makes sense. Also naked mode post inferno AND naked PvP will really give player bragging rights, as to win requires skill and not gear which can be purchased for real money, which gives even a 3 legged cat an advantage over a "good player". Beating the naked mode could give rewards like extra stats when completed, again giving players endgame.
Not sure if this will fix anything, but it should be implement. You should have the ability to custom fit gear, renaming it, and possibly adding something, like a socket for the cost of crafting material relevant to the gear your upgrading. (customizing iLv63 gear would require inferno materials) I don't see how this ruins anything because currently, not even 2 months after release, there is an ABUNDANCE of gear.(check the AH?) Also a player would have the option of customizing his gear, just like they have the option to salvage it. Small upgrades to weapons is what people want from the blacksmith, not the current "gold sink" that he is. Also the ability to name your gear would really go a long way in giving people character identity since currently there is no differences between to lv60 heroes since they can respec everything and potentially have virtually the same items. Perhaps an option to permanently set items into your slots, giving you some sort of extra buff, again giving heros some form of identity, and a reason to re-roll a hero. For example set a weapon permanently to your hand and you'll get +5% dmg and perhaps a bonus element applied/added to all attacks (even if the attacks do elemental dmg already). Maybe setting boots will give you +10% Movespeed? This idea may just piss people off but, again it adds endgame and replay value.
You could implement an entire new class of weapons. I'd like to see hot pink text for them, orange is almost hard to see sometimes. The new gear would be basically temporary. The gear would vanish or lose stats over time, but be SLIGHTLY better than all the gear in the game. These items could be heavenly/hellish/ancient/possessed items that either had better roll chances, or unique stats. This would be somewhat complicated to implement but give the endgame options, like reacquiring one of these items.
I think the best thing to combat botting is to limit the amount of gold transferred by IP and account. Also implementing a NV for gold, mandatory from lv1. The idea would be simple, you'd have to kill a certain amount of mobs per min/hour to get current 100% gold drops. If you just broke pots you'd get like 5% of the normal gold drops, basically destroying current botting trends. This would be somewhat unnoticeable to normal players but would almost destroy gold farming bots, until they're remade to adapt.