things they should fix in the upcoming DLC

add a place for us to store a huge amount of items

with so many wepons and mods why would you not give us a place to store things we want to keep but not carry with us and take up our backpack

Storage is important, at the very least. Giving us that many items without giving us any storage is either an oversight or simply something they were unable to provide due to time/space constraints, both of which I'm definitely willing to understand. Vehicles could be a little updated, if they have time to fix that at all; I think there's already a thread devoted to that. New guns are probably going to come, though I'm more than happy with what we've got already.

More classes would be EXCELLENT. The awesomeness of Borderlands is how you need to play a first-person shooter like a Group-based MMO, and having more classes only adds to the different class combos, builds, and wonder of the game. If they do give more classes, then they're going to have to develop them one-by-one, and release them individually, I'm thinking. I'm absolutely sure they have old class ideas to work though and see if they can bring foreward. Fitting them into the game, thematically, is going to be difficult; there's a very, very small number as far as "people who can fit into the back of Marcus' bus" at the very beginning of the game, and they'd need to come up with a whole different class dynamic to build off of. Maybe they'd release new classes in whole world-expanding DLC packs in the same vein that Diablo did; that brings with it the danger of introducing a completely new dynamic to the game and then having the new class rely upon it entirely. This has the effect of making that one new class NECCESARY, limiting the importance of the other classes, screwing up the balance. At the same time, it would also make the new class a little bit less awkwardly introduced, which is always a welcome change, and if this new class does require the new dynamic and it screws up the game play that much, I have the option of not purchasing the pack. That, and I doubt that Gearbox is going to make that mistake anyways, given how awesome a balancing job they've done with this one.

Cosmetic changes would be nice, occasionally, other than changing color pallete. Mordecai without the mask; Roland with a helmet and fuller armor; Lilith most likely with something more revealing, as befits her nature; Brick with a mohawk, no shirt, and tattoos. I wouldn't pay for them, but they'd be pretty spiffy. Look at all the cosmetic changes you can get in Halo, or even that BlackOps downloadable class-based shooter for Xbox Live Arcade (give me the real name if that one is wrong!). Just some minor add-ons, if a lot of them, enough to really make your character feel like -yours-.

Or even some characters in the same class with different backgrounds. Playing a feral, psychotic Tank with bones in the hair and piercings everywhere, for example. A female Hunter based on the whole cowboy thing, with bandannas and stuff complete with gold star badge, or maybe even a more agent-based Hunter, with more black-ops looking gear that uses something else that acts and has the exact same effects as Bloodwing, but looks different (a flying robot drone, for example). A Siren who pulls off the whole corperate look, with a suit and tie and trendy shades and high-heels. A Soldier who is also an escaped prisoner, wearing an odd mish-mash of convict garb and riot cop gear. Even gag characters, y'know? Make the hunter look like T.K. Baha instead, or the Tank look like Marcus.

There's a lot of ways this could go, honestly. I'm really, really excited to see where... but for now, we'll just have to deal with the cards we're dealt.

Get rid of the duping, it'll kill the open community.

Some kind of trading system so you can give people MONEY and safely trade items in random games.

Some kind of additional storage, you are going to find stuff you don't want to sell even if you out grow it, and you'll find a buttload of class mods and artifact (which are one time use) so you'll want to mix and match... all of that take their toll on the backpack.

A rolling system option that the host and put on in the options to cut down on ninja looting and item disputes.

Also if a high level character joins a lower level host, the higher level character should be able to collect quest items and participate in turning stuff in.

There are others but those are the biggies.

"Get rid of duping"

As if it's that easy.  I've seen it done and honestly, I can't think of a single way it could reasonably be fixed.  Unless the game is constantly saving every last change in the inventory, it can't be fixed really.  

I'd just like to see more cosmetic options and maybe a new vehicle or 2 (like those Crimson Lance ones that are never used).  New costumes even would be appreciated.  

If they were to add (a) new class(es) I'd like to see one or two new weapon classes too.  Obviously this new class would need some weapon to be awesome with.  Gatling guns, flamethrowers, grenade launchers, more Eridian weapon types.  

It's not that the option isn't there, it's that it was broken.  I believe someone posted up here that it was fixed by now though.