1)Melee Weapons
One thing I felt was missing - especially for Brick - was the ability to have straight-up melee weapons equipped. Brick is the perfect class to be the expect w/ these skills, since he is a berserker and all - and can lay waste to people up close and personal w/ his bare hands. Let's have some baseball bats, axes, frying pans, etc - yes, it did wonders for having Melee Weapons in L4D2 and there were classes in HGL that specialized in this - BL should join the club.
2)Crafting Weapons / Disassembling Weapons For The Parts / Upgrading Weapons
I know the dev's were asked about this - and they left it out due to the fact that had insane amounts of weapons in-game. One of the best features of HGL was that you could make weapons, based on the "recipes" you find and if you have the "correct" parts. Also, you could break weapons into their pieces and do what you want w/ the parts - maybe make another well; sell the parts; give them away to another player; etc. Also, like HGL, Torchlight, and many other ARPG's - there should be socketed item slots - where I can upgrade my weapons by adding runes and gems and stuff to them if I have the socketed-item slot. Why not allow this kind of customization? Cheaters already run rampant w/ modifying the weapon game-files on ALL of the systems anyways, so let them be able to customize stuff in-game...
3)Vehicles - deeper vehicular combat / customization of vehicles / more vehicles types
Originally, BL was to have more different vehicles and a little bit more customization than what's actually in the game. Some of this incomplete code has been locked-out of the game - and people been messing w/ it. Due to time constraints and that they didn't have more time to fully-develop it, it was left on the cutting room floor. Let's expand on this, Gearbox.
Rumor is - an upcoming Borderlands DLC might be focused on vehicles entirely.
4)Improved Optional Storytelling Methods
The storytelling, dialogue, and presentation of this is by no means bad in Borderlands, when it's done - see the opening video and some of Tannis' journals. Also, see their Brothers In Arms games, too - it's extremely well-done. The thing is, they didn't want to slow Borderlands's action-game style pace down for the action players - especially in Co-Op mode - where players are always voting to skip cut-scene sequences, which a lot of players do all the time Guild Wars. It's just...I think they really could've used optional ways to convey the story more so.
Here's my thoughts - employ some optional storytelling devices. Think like a Codex system like DAO, The Witcher, or even (a voiced-one like) Mass Effect has to give the player more additional information about the game, the gameworld and its characters; more Bioshock style audio logs to pick-up in the gameworld to give the player more info (some of the best storytelling was Tannis' journal quest); and even maybe video logs for the player to pick up like in Dead Space. Hell, there could even be Achievements for players finding all these audio logs, video logs, or whatever in-game - making it even very worthwhile for MP gamers to go find all of these things in-game together - even if they don't look through them while they're playing online.
Likely, most players would NOT dive through reading, listening, and absorbing all this optional storytelling stuff during MP; they'd do it during SP mode - this would really satisfy the gamers who want more story out of their game.
I'm sure there's others things I will probably think of later, but I'll do it in another post...