Thought i'd add my 2 cents as ive been playing all the Fallout's, (even Fallout Tactics) and it would be nice to see some things I would like in the game:
The Playing Character have a genuine story line linked back to a previous game, as in both FO3 and NV, the PC has been a new character, with little to no history from the game itself.
Have a longer story line, as I felt that both in FO3 and NV the main story is quite short, and the way they have pushed this is to included alot of side quests. (Which is good, just my vision and preference is to follow a story line with many different variations of scenarios to acquire or fail)
Be able to setup a base from either acquiring a town/village/houses from NPC's (Both Evil Junkies/Fiends/Raiders and Good people) and make this your home with this own name, with its own story/history (make it up or follow a previous legacy) and have raiders attack it, or have caravans trade, settlers setting up shop, recruiting guards etc etc etc..
With linking to the above, if/once you have a decent sized settled village/town, you can setup a raiding party (or become the leader and lead them yourselves) to either help others in need, or raid other towns and settlements, or setup your own trading caravans...
The above 2 is also linked with its own reputation, and once the game has been completed, you can move on, destroy what's been built, or force an alliance with some other group. (I think you see where im going with this)
Better modifications to both weapons and armour alike. Wether its stitching on leather shoulder pads to a vault suit, or formal suit, to duck taping a combat knife to the bottom of a shotgun to provide closer range combat for the "OH SH**, NO AMMO" moments..
More and better interaction for them chillax moment, i.e. when sitting at a bar, have people talk to you in general chitchat without the stop time situation.. Better and more genuine interactions with NPC's
Multiplayer mode - have 2-4 people co-op mode or have a mode of evil, rather than being the saviour of the wastelands, how about becoming a Raider Leader? be apart of it from the other side, even at the start have a companion from the start, as if it is another Vault leaving scenario, have the dialogue to have someone follow you (a friend maybe) or demand they stay in the vault) etc.. This reflects down the line on the story, and they can be killed and again, this affects story line..
Makeshift vehicles, surely by now, people would have used the tools required to come up with some sort of travel rather than walking.. Im not talking "Pimp my ride" style either
More side games, boxing, full blown fist fighting, a hunting game (Like the movie, think its called the game) were people have the same gun, armour and the last one who lives..
Character creation, rather than a default body, surely there is something we can do to create someone of a Very Thin, Thin, Average, Bulky, Sticky, Fat (These reflect in the SPECIAL stats), with this, there are different interations within the game, as in you can become a bouncher at a bar or a guard if your the stocky type, or you may become a town doctor if you have a high enough medical/doctor skill/trait.
Have preset builds, Warrior, Fighter, Scientist, Gun Slinger, Geek, Hunter - These are built in preset charactistists but can be change thought-out the course of the game to reflect the actually players type of RPG experience.