Sina's opening scenario sounds more like the Borg than power armour, with all those red beams cutting from the dust.
Jumpjets? What you want is a
rocket pack though to be honest I can't see the need for a working one unless it's used as a device to get you to a previously unreachable area like with the tanker.
What would be nice is to walk into the junkyard and have a rusty old Rocketman style helmet and pack in amongst the junked cars. They thought we'd all be flying around on rocket packs by now, so it would be nice to see some remains of old tech other than just cars.
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I'd like for FO3 to move away from pre-war tech, set it a further 80 years on and have the old technologies winding down and wearing out. Not just high tech like laser pistols, but the other pre-war stuff like combat armour, AK-112s etc. Other than new versions that might of been manufactured by the Brotherhood or maybe the Vaults most of these guns are going to be several hundred years old, they've been through a lot and are bound to become unusable in the end.
There should be very few if any books around, most of them should of fallen to bits by FO3, those that are left should be horded not left scattered around. The amount of people with knowledge of old tech should be few and far between. The brotherhood should be a scattered bunch of old men without a functioning suit of PA between them.
There should only be one suit of power armour available to the player, the T51-b. It should be treated like a vehicle, no addons other than perhaps a headlamp/nightvision mode. And when the player finds it, it should be in a junked condition and you'll need to find spares, someone to repair it (or to teach you to repair it) and a power source. I say no addons because being a walking tank ought to be enough of an advantage over the other armours without any fancy retractable blades or chameleon circuits.
As for the face of the Wasteland, I'd like to see more fortified towns, the NCR should be gone, New Reno an abandoned ruin. All the vaults that are going to open have done so and the rest are tombs.
Instead of the tribal, raider, slaver etc definitions people should be divided between settlers and nomads.
Nomads would include herders, merchants and ferals.
The herders would be the closest to FO2's tribals, they have little time for education they spend their time driving their herds from one pasture to the next, living off the land or trading animals for grain and other goods.
Merchants, just like in Fallout but rather than having an office in the hub and little car trunk carts they have great big cavarans (converted trucks or rail cars?) pulled by teams of brahmin. Which are as much home to them as well as cargo space.
Ferals, small groups, wild, lawless. Either orphaned and banded together for survival or kicked out of a settlement or nomadic group for crimes committed.
The settlers as the name suggests live in settlements across the wastes, most are farming communities, with a few ranches (though they are always in conflict with the herders) and some specialist communities producing a single item (like booze).
The largest and richest are descended from the Vaults and like New Arroyo were constructed with a GECK. They found a suitable valley with a water supply, walled off the entrances and exits, to the valley leaving just one or two gates to allow passage. Using the instructions with the GECK they built a water purifier and irrigation system. They test their children for learning aptitude, those who pass go to school, those without any aptitude go to work, menial at best. Their fields produce the largest, healthiest crops but they need a lot of workers. Most other communities are built amongst the rubble of the old world, but they all keep a trained militia for protection and have some form of law.
There's no real clear division between citizen and raider, usually the settlers and nomads live on barter and trade, but if they sense weakness and opportunity they'll raid each other. The nomads don't have a need for slaves, but amongst the settlements especially the larger ones the need for workers has caused the resurgance of slavery.
I think that should be one of the main themes, of FO3, the fight against (or for) slavery. Not the Masteresque plot, but your starting point. Start the game after a slavers raid, perhaps having the player start as a slave.
You could build up to a war between those communities against slavery and the slavers, with the player getting to chose sides.
As for the remains of the Enclave, any troopers that might of survived would probably return to Navarro to regroup, it's doubtful that any were in on the full plans of the Enclave. Most would of been brought up on the ideas of the US. It would be interesting to see them form a Brotherhood like organisation, but dedicated to re-instating the laws of the old US. Perhaps joining up with the remains of the NCR Rangers and calling themselves Marshals? Fighting against slavery and for truth, justice and the American way of life (or however that goes).
Eitherway they should lose most of the tech, any still working APA should be almost worshipped, kept on display as a symbol of what the old world could achieve but never used except in the direst emergencies.
Ghouls and Super Mutants should be few and far between, between Fallout and Fallout 2 bad endings for Ghouls there can't be that many left. While there should be still some around it would be nice to see some more general mutations amongst 'normals'.
Deathclaws were much better in Fallout as a shadowy monster that everyone's afraid of but few have actually met. Good for a mystery, but not a main plot.
Just my tupence worth.
Cheers