What do you want to see in the game? Perhaps some references to the events of the previous Fallout games?
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1. Vehicles. The wasteland is scattered with broken-down cars and vehicles. You're telling me that absolutely NONE of these are functional? That makes no sense. Show caravans with modified army trucks, allow the player to fix up a motorcycle as a mount, just do something. It annoys me to see all these vehicles that LOOK completely functional just lying around, and walking around the wastes takes WAAYYY to long. 2. Property. The player should be allowed to buy (or take) property throughout the wasteland. One problem I had in Fallout 3 was a lack of unique property. I'm pretty sure everyone just kept their stuff in the Megaton house (or the personal vault, if you played on PC and knew what FO3 Nexus is). 3. Dynamic Factions. The role factions played in FO3 was minimal. They had almost no impact on how the game world was perceived, and outside of faction related quests, there was very little even acknowledging their existence. New Vegas tried to fix this by allowing you to help either Caesar's Legion, the NCR, or the "Mr. House" faction. However, this system still wasn't all that great. Yes, it made the factions have a larger impact on the story and the game world, but it still didn't feel very interesting. If there we're four or five factions who had varying relationships with each other, politics could play a large part in how factions and their members interact with the protagonist. 4. AI. The AI in Bethesda's games is probably the stupidest AI I've ever encountered in a videogame. Hopefully this issue is fixed in future installments. 5. More customization options. Allow player to equip more than an outfit and a helmet. Take the Elder Scrolls route and allow players to choose their boots, pants, shirt, and head wear. This would drastically increase both immersion and the player's connection to their character. Unrealistic expectations are unrealistic. Regardless, the reason Bethesda games are so great is because they actively promote modding. Everything aside from my second point could be added in a mod (and who knows, even that may be possible.)