One of the aspects of an aRPG is that it serves as a glorified stamp collection; and the worst that can happen to a collector is that he finishes his collection. So, implementing a tremendous amount of variety for weapons would allow players to loot for a very long time, increasing the endgame.
Rare weapons, like in D2 - weapons that you don't find much, but spawn 4-8 (or more) modifiers, generally weaker modifiers than the ones you get on normal magic weapons. If you're lucky enough to find one with 8 modifiers, all being powerful (which should be rarer than the weaker affixes) you'll probably have the most powerful weapon in the game.. but the chances of finding the perfect rare would be negligible, especially if all the powerful affix also spawned in perfect form.
Magic affixes being the most powerful affixes in the game; because you can only have 2 affixes, as opposed to uniques and such, they should be more powerful, so even they remain interesting, especially for players who focus on extremes; like a player who thinks its worth sacrificing all his health/resistance/dodge/etc for a huge damage output. Petmancers, for example, as they can stand in the back while the pets do their work. Perhaps even allow low level aura's as affixes... Ofcourse, the powerful affixes should be pretty rare.
More affixes! Some ideas for affixes;
Grant skill (improved by +skills and usable by all classes - may also be unique skills that don't exist in any of the masteries)
+3 to specific skill (only usable by classes that own the skill)
Level 6 aura (fixed, not improved by +skills)
Reduce health by 15% for target as well as user
1-3 sockets (varies - makes for an interesting situation if this spawns as a prefix and affix, sorta allows players to forge their own weapons)
I should make a thread for affix ideas if there's interest, there's a lot more stuff to come up with
Auto-mods; another d2 idea; specific weapons (like the staves) could drop with integrated mods, like staves being able to spawn 1-3 specific skills that give 1-3 skillpoints for that specific skill. Perhaps only put automods on weaker versions of the weapons (lower damage output, slower, less blocking, less armor, etc...).
Lots of varieties in affixes and unique weapons; so that finding a specific affix or a unique you were looking for still allows you to find a better version. The more variation, the better.
MI's were pretty interesting - it gave an incentive for the player to go back to certain monsters, instead of just farming bosses 24/7. Ofcourse, some MI's had crap bonuses, so a useful bonus for each MI would, that would set it apart from other MI's, would be great. Perhaps nerf out the most powerful affixes though, so they don't 'exclude' normal magic weapons from the game.
Scrolls being carried and perhaps even used by enemies; I'm not sure if there are going to be scrolls, but it'd be interesting to equip them on monsters; I bet they'll be seen as a funny and rewarding addition and they'll be used a lot more - especially if you can't sell em or fetch only a moderate cash price for it.
Rare stones with unique abilities;
D2 had very rare runes, granted, they overdid it with anything of ist or higher; but it still felt really rewarding when you found one of the higher runes (never got one above Gul, chances above that are abysmal). TQ however, spams relics and charms, which makes them much less rewarding. Plus there's not a whole lotta things you could do with them... So, having very rare stones which open up a new aspect of the game (runemaking, crafting, I dunno...), opens up more room for infinite looting
Put several statues in the game, which are broken into 20 or more pieces; the more pieces you get together, the higher the price. But if you're in need of quick money, you can sell the thing for a lower price. Only really useful if the economy is fixed though...
Oh yeah, and fixing the random issue would help too