I've played this game entirely self-found and single player since release. Never clicked the AH tab, never clicked any kind of multiplayer option. Actually, I think it's pretty close to what I would want overall, assuming we don't make any large systems changes. I have all 5 classes to 60, and am farming various parts of Inferno with them (MP1 for most, MP3 for my WD, which cleared 1.0.4 Inferno).

The things I would want in a single-player mode (again, assuming no huge content changes) are as follows:

- Offline mode, obviously.

- A "save state" / "restart state" mode. NV stacks aren't a bad idea, but right now I'm not going to play D3 if I only have 15-20 minutes, because the farming is relatively inefficient and I will just feel pressured for time rather than relaxing. I think there should be a save-and-quit mode that preserves mob deaths / your NV stacks / etc., but there should also be a "reset the game" mode such that you can repopulate areas when you're ready to.

- A "challenge" setting that works as follows: you set a number of deaths you're allowed before the game boots you and repopulates everything.

- An additional challenge setting that saves state every few seconds. When you die to an elite pack, you have the option of restarting from before engaging that pack. That way, I would be able to see whether or not I could actually kill a pack fairly, rather than whittling them down with one lifespan and cleaning up on the second. This isn't as big of a deal anymore, but I really wanted this a few months ago, when I wasn't really farming anything, and didn't want to feel like I was cheesing down packs, but had no choice, since no such option was available.

- Double the current drop rates for legendaries and set items. Right now, I get a legendary or set about every 2 runs, which to me seems a little low because a large chunk of them aren't usable. If I got about one a run, that would be fine -- I could look forward to those rather than the incremental rare upgrades.

- Overhaul crafting to remove the dependence on the crap droprate for patterns. Specifically, for gems, you should be able to upgrade to the highest quality gem after crafting some large amount of the lower quality gem (i.e., craft 50 star emeralds -> unlock the next level of emerald). However, when unlocking, your Jeweler would only know an inefficient recipe -- it would cost more gold, more star emeralds, and maybe some essences, etc. However, every upper level of design you craft would have a small chance of unlocking the cheaper, efficient recipe. Something similar should apply to the blacksmith (4-prop, 5-prop, 6-prop plans), but after you're crafting the 6-prop plans you should be able to have a chance to unlock designs for awesome legendaries/sets that are craftable -- a reward for dumping so many resources into your blacksmith.

- Rebalance the stats slightly to make resistances matter less in the grand scheme of survivability, and enhance the effects for str/dex/int contributing defensively for all classes to make up the difference. Right now all res is an incredibly annoying stat to get on self-found items because things just don't tend to roll with usable amounts of main stat/vit/all res all that often, and I'd rather wear something like 200 main stat / 100 vit over 80 main stat 80 vit 80 all res, which is what tends to roll most of the time if those 3 stats are even on a given item.

- Endless dungeon. There are tons of ways to do this, but I'd have fun with it no matter what, so I'll just leave it at that.

- Make whatever other endgame content not dependent/easier to do in groups. Fighting Rakanoth is already annoying with most classes -- don't make me fight him and Ghom together in a game. This is probably why I won't bother trying for a Hellfire Ring -- not worth the effort as everything about the Infernal Machine event is clearly geared towards groups.

- Add additional daily/weekly/whatever challenges for defeating certain mobs/elites/bosses/whatever with certain skills, runes, passives, and so on. Add additional challenges for using a randomly-determined skill loadout and completing some task.

- Add a system akin to badass ranks in Borderlands 2, tied into achievements. Achievements would either give you account-wide or character-wide boosts, depending on the achievement (very marginal), or cosmetic novelties. This could all even be tied into the banner -- banner shapes, trims, and stands are account-wide and offer specific buffs for all characters, banner sigils, adornments, and accents (blah blah, add categories of banner upgrades as necessary to balance) offer character-specific improvements, etc.