And it's time again for my regular suggestions.

hardcoe Mode +1

• H20, SLP and FOD meters go up three times as fast.
  
• Radiation intake is tripled (Or maybe just x2.5)
  
• Doctor Bags cost 500 caps
  
• Doctors cost 500 caps to heal your health, radiation or addiction level. (Doc Mitchell will do all three for 250 caps but only heal limbs to 50% and only remove half the
• radiation)
  
• If you don't meet the weapons soft requirement then you're going to sway alot more and reload slower despite your Agility stat if you don't meet it.
  
• If you don't meet the Strength Requirement then reload speed should be much more slower than it currently is, weapons sway more and recoil is like crazy.
  
• Caps have weight.
  
• Healing items (Chems and stimpacks) have weight.
  
• Repair vendors charge twice as much as they currently do.
  
• Weapon mods' price is increased by five times.
  
• Ammo weight is tripled (excluding missiles which already have a ridiculous 3 weight.)

And then hardcoe Mode +2
All of the above but also with:

• No fast-travel.

Please add in assassins or bounty hunters for each faction if we piss them off.
Like these:
Powder Gangers.
NCR.
Legion.
Khans.
Houses Securitrons. (Yes they'll go outside of Vegas/Freeside.
Kings. (Yes they'll go outside of Vegas/Freeside.
Van Graffs. (If you slaughter the store then Jean Baptiste and Glorias mother will send her own assassins after you)
Nightkin. (Black Mountain, kill them off then you'll get Nightkin after you/Kill Keene)
Fiends. (Yes add a reputation bar with them, I know it can only go down.)
Boomers. (If you kill Pearl and Loyal)
Brotherhood Of Steel. (If you kill even one of their soldiers)
Gun Runners.
Crimson Caravan.
White Glove
Omerta.
Tops.

I know it's a long list but I really think there should be more to it if we piss off a faction.

Extreme Mode.

A non-canon mode which gives the enemy (or ally if you side with them) incredible equipment.
Sure the PC could get it as well once one kills a hostile NPC but it would still be a challenge.
Maybe just up and remove things like young gecko's or purple gecko's and molerats and replace them with something more dangerous like big packs of nightstalkers.

Just imagine how hard it would be at early levels when Powder Gangers run around with missile launchers and plasma grenades.
Or how NCR Troopers are all of the sudden armed with sniper rifles and Assault Carbines.
Or how Legionnaires are armed with stuff like Super Sledge and Ballistic fists.
Or how Nightstalkes at early levels have 5 more in their DT and their poison twice as dangerous.

Hey Obsidain, mind adding a new faction in the wastes which use completely random weapons and start showing up at lvl 20 around a lot of places in the wasteland?
From 9mm pistol to grenade machine gun, these guys and gals will use everything. (Except mines, I don't know if NPC's can plant mines or how their AI would work with that)
I wouldn't mind it if they had about as much info as Vipers have in the game (none) but it would be better with some new dialogue or holodisks explaining a teeny weeny bit about them.
This faction could really switch up the gameplay.
One time you're meeting four of them with knife, sawed off shotgun, plasma pistol, flamer and the next time you might meet eight of them but this time with knife, riot shotgun, scoped hunting rifle (Yes, these guys are allowed to use modded weapons as well), plasma grenades, brass knuckles, grenade launcher.
It would mean that sometimes these guys can pose a real threat.

And if not completely random, how about them spawning at lvl 1 but gaining leveled equipment alongside the player?
So that by lvl 30 the only items they will use are the top of the line weaponry like multiplas rifle, grenade machine gun, ballistic fist, AMR, super sledge etc.

This faction would also wear either random armor (if going by the first example) or leveled armor.

Where should they spawn?
These guys take over other npc's and creature spawns temporarily.
So for example, at the first dry lake there are tons of giant ants.
But when you go there you might meet this faction who has replaced their spawns.
After you kill them and wait three days the spawned creature will once again be giant ants.
If this is too complicated and can't be done then just add at least 30 new spawns for these guys around the wastes.

Don't get me wrong.
I don't want this faction to be it's own DLC.
But rather have this faction be included as a bonus with Dead Money or Honest Hearts.

One problem I see with this is that we might have it too easy to earn money by killing them.
So how about this, these guys have explosives on them which are rigged to a small heart monitor so that when they die they automatically blow up into tiny pieces and all equipment they have will be lost. (I think I saw this on a Lost episode.)
Why they do this? Maybe they don't want their enemy to get the upper hand if one of their members were to die.
You can still pick-pocket their stuff though but they have a raised awareness so it might prove more challenging than with regular npc's.

This faction might actually be a faction that is challenging to meet.

Obsidian, take a look at this thread: http://www.gamesas.com/index.php?/topic/1148266-they-shoulda-been-companions/
If it's possible for you (with the voice actor/lore/character/???) to make some of them companions in a DLC as a bonus I'd think we'd all appreciate it.
(I know they need their own storylines and all but you've got great writers so that shouldn't be a problem right? )