• There would be an option to switch back and forth between a D3/WOW stylized graphics option, and a Classic Diablo look.
• Players would create Diablo 1 and Diablo 2 conversion packs, also with randomly generated maps.
• Players could sell these self developed maps on the D3 MapMarketplace.
• If Blizzard created a Diablo 1 mod, it would be called "D1 Phoenix". :)
• A player would create a "World of Diablo" mod, but they would be forced by Blizzard to change its name to "Diablo Universe".
• Players could swap fonts or select new color schemes for their resource bars. For example, Monk's Spirit could be green slime instead of white spooge.
• Even faster quest pickups. The player would just need to run within 20 yards of a NPC, the quest is auto accepted, and the menu closed.
• Yards are converted into meters for the confused in other countries.
• Diablo 3 Auctioneer for much easier management of sales, and tons of undercuts. Meaning more profit for Blizzard and the powerhouses of profit would be making undercuts and reposts constantly.
• Enchantrix like mods. Mousing over on items would bring up tooltips showing disenchant results as well as converting gold value into current market value cash conversion.
• More skill keys and the ability to swap in and out of pre-programmed builds quickly.
• New unit frames, and customizable party frames for easier management.
• Floating combat text that shows XP, Gold, or Cash wins upon finishing off an opponent.
• Larger combat text with customizable fonts ("explosive fonts" like out of an arcade fighting game).
• Recount mods. For threat meters and tracking all statistics that Diablo 3 doesn't. For example, mob kills, highest swing damage, largest heal, pots drank, cash spent, etc.
• Lore book UI integration with "D3 Recount" showing interesting statistics about each monster. Total kills per act, average level, most amount of damage done, most damage received, value of loot drops, cash value of loot drops, etc.
• PVP Support mods, for monitoring DPS meters, cool downs, quick build selections, debuffs, onscreen timers and countdowns, etc.
• Automated in-game gambling systems like trivia bots, card games, or roulette tables for real money.
• Better stash management and better Auction House UIs
• Financial management mod for showing gold/cash per hour, or will lock you out from buying items (only selling.)

Of course, some of these mods would be considered unauthorized (eg. swapping media content) but I'm just lumping everything together because it's all just fun wishful thinking.