I'm mostly happy with the game as-is, but here are the changes I would like to see, and they would work best by adjusting the base game as well.
If I had to choose one thing: Split the quests into main quests and side-quests or events (as per Titan Quest and other games). It's fine if we have to do some quests at key points, but it gets extremely annoying having to do the exact same quests in exactly the same way every single time. Generally speaking, you'll want "kill boss X and progress pass it" as main quests, with everything else as side-quests. The bgigest offender of this key problem is, in my personal opinion, the catapults in Act 3. Splitting this up would allow us to play the game far more freely.
- Increased outside randomization. A lot of the game is spent in outside areas, and a lof ot outside areas are just too similar in shape that even the randomised events and set pieces don't do make it random enough that. Once you've been through, you know exacly where to go.
- In extension of the above, alternative paths. This ties in well with my primary choice of what I'd like to see in the expansion.
- Increased atmospheric randomization. Lighting and sound can change the mood of any scene significantly. That same damp cellar looks very different if there's a purplish mist and magical swirls than if there's a red-yellow hue with dust particles and a heat shimmer. Simply create
- In extension of the above, a day/night cycle - IF this can be done well. It will increase replayability and feel more random if we're seeing different areas of the game at different times. It's still a game, so it doesn't have to go pitch-black.
More depth: By depth in this context I mean depth in the map. As it is, I can /see/ my characters occasionally moving up or down a little bit, or going down "into a cave" or up "onto a set of stairs", but it's followed by a zone change, which rather kills the sense of depth - it all feels very flat. This is even true of what is currently the final act, where there should be a better sense of increased elevation. I'm not really sure how to work on this or what would or wouldn't work, but it might be worth looking into.
Gear: Additional options, such as enchantment. I understand that you feel that these don't really add anything per se, but I disagree with that assessment. It's fine to have some double systems in the game. I'd suggest something along the lines of crafting, but you might want to work on the potential inventory issues for this. On that note...
Increased inventory: It really bugs me when games bring out expansions and do not increase your ability to store items. Yes, I am a WoW player with many filled banks, and a grudge. :-P Might I suggest a small personal storage chest for each character, in addition to the shared stash? This is especially useful because it lets us collect gear for our alts without it taking up inventory space during playing. It's very annoying collecting good pieces of gear for an alt, transferring it to them, and having your Lv13 Barbarian play with only one-third of his inventory free.
Zones: Definitely, undoubtedly, a jungle-y area. Of all the zones, I'd really hoped this would have been the one to make it into Diablo 3, and I was really disappointed it wasn't the case.
Increased zone activity: Zones feel really dull. It's essentially a hero smacking demonic monsters. There is some REACTIVE activity in the form of breakables and chandeliers and such, but what would really make the game more exciting is an increased amount of ACTIVE activity. Think "boss special attacks", without the boss.
Collector's edition: Another epic collector's edition, with a ton of extras. I love my Diablo 3 CE. <3
Fishing. Haha, I kid. It doesn't really fit into this game. I enjoy it in Torchlight, though. :D
I'm sure there's more, but that's what I've got right now. :)