The first thing to know is that fall-proof and bulletproof are largely unrelated. Sure, if you put enough "durability" on anything, you'll get both, but if you want to optimize your modifications, I'd focus on one or the other.
Optimizing for Bullets
Bulletproofing is fairly straightforward, as it largely comes down to three basic ideas:
- Distributing kinetic energy across a wider area
- Absorbing kinetic energy (typically through plastic deformation)
- Preventing penetration
The first and second are reasonably interchangeable, where more capacity to distribute force largely obviates absorption capacity, and vice versa. If you can distribute the force of a bullet from an area less than a square centimeter in size (bullet cross-sectional area) to around 2,000 square centimeters (approximate torso cross-sectional area), you've reduced the kinetic energy applied per unit area by 2,000. If instead, you put a wall of concrete in front of you, that doesn't do much as far as distributing force, but it does wonders at absorbing kinetic energy through plastic deformation.
Preventing penetration is largely about putting fibrous materials that will "catch" a bullet before they enter the body, and these same types of fibrous materials (Kevlar, for example) tend to do distribution fairly well.
So some fibers for distribution and prevention, and some ceramics or other plastically deformables for absorption is the typical solution. Note that out of the fibers and deformables, the deformables will help somewhat with fall resistance, but the fibers will not. Which brings us to...
Optimizing for Falls
The first question to ask is what it is specifically that typically kills people in high-altitude falls. I'd imagine it's breaking of the spine, and the bones of the ribs being pushed through the organs. What causes both of these problems is the same two issues: things bending like they aren't supposed to, and jerk being applied to the body.
To stop things from bending, you can include a frame of some sort, either directly to the spine or to a frame outside the body. To stop the jerk is trickier, and comes down to either reducing the force (either absorption like with the bullets or decreasing speed of fall) or slowing down the impact (think a giant pillow, gently bringing you to rest upon impact). Here you have a few options:
- Add means of drag or decrease weight to decrease terminal velocity, for example, wings or a parachute
- Add plastically deformables (see bullet section for more on this) to absorb the force of an impact
- Add cushioning to slow an impact
Here your creativity is the limit. Plenty of real-life creatures can survive the force of an impact at terminal velocity. For example, most insects cannot be killed by falls from any height, because as you scale down objects, they tend to handle impacts far better. A human-sized ant would be only slightly more fall-proof than a human-sized human. So small size is your friend.
You can also add something like a biological parachute or wing, which can slow you down just before landing. Of course, this isn't as "cool" to a reader as an enhanced human who manages superhero landings.
Another interesting option is crumple zones, areas of your enhanced human specifically designed to smash, absorbing the force of the landing, perhaps quickly healing/regrowing after smashing. You can also try to capture the kinetic energy of the landing, with some electromagnetic damping or something similar "capturing" the force and generating electricity, glucose, fuel, or something else that can be used later.
Finally, cushioning. This one is tricky, because generally you need a lot of it. Crash pads for bouldering (low-altitude rock climbing) are typically around 13cm (5in) thick, and that's just for heights below 6m (20ft). From that height, you'd only be able to reach around 33km/h (20mph) at maximum, disregarding air resistance, well below the estimated 195km/h (122mph) terminal velocity of a skydiver. Basic physics dictates that kinetic energy, the real problem with falling, grows with the square of velocity. This means that if a 13cm (5in) mat can handle a fall at 33km/h (20mph), a fall of 195km/h (122mph) will pack not 6 times the punch, but 36 times the punch. If required mat thickness scales linearly with velocity (which it doesn't, it's probably even worse than that, so this is probably a best-case scenario), that means you'd need 78cm (31in) of pad to take an impact at terminal velocity. Again, this is only a best-case scenario, so you'd likely need much more than that, and 78cm (31in) is already a fairly prohibitively large amount of padding to add to your enhanced humans.
If you want to stop bullets, add fibers and plastically deformables. If you want to stop falls, slow down the fall, add plastically deformables, or slow down the impact with biological padding. Let me know what you come up with! As far as creativity goes, the sky's the limit. Then again, that's sort of the point :)