1. A real hardcore mode. Make me get minor sleep deprivation in 24 hours with an ever-increasing rate of greater debilitation beyond that, make all ammo weigh twice as much as it does now (I can still carry over 2 thousand assault rifle bullets, with 3k minigun shots, all the water and food I need + combat armor at strenght 5), make weight carried seriously increase rate of hunger and water deprivation, make hunger deprivation increase in stage at least 3 times a day and so forth.
In essence, make it matter something. Seriously, people lugging stuff like that in real life would consume more than just a few iguanas on a stick per day, with water expenditure on desert measured as bottles per hour.
As it is, the 'hardcore' mode is like 'normal' where non-hardcore is like 'tutorial'. If you're going to call it hardcore, make it at least slightly challenging.
2. Expand upon inter-tribal relations, like the Khans, the Fiends etc.
3. Add perhaps as a new questline NCR influx to the area.
4. Expand and deepen the Strip faction questlines.