the PC wakes up on a beach clueless as to who they are and what they are doing there.
No more Zelda for you. I think that there needs to be some driving incentive to wander the wastes.
The time setting is about the same as FO1 maybe a bit earlier.
A prequel wouldn't work, games have to move forward. The only good reason for a prequel, is to answer character questions, however since Fo1's main character lives in a Vault, there is no need for questions.
One of your quests could result in a vault opening.
The vaults were made to survive Nuclear attack, the nuclear assault is over, so there is no point to a post-war vault.
Ultimately your task is to find out who you are but the answers may come from an evil rising in this area. You can get answers by joining them or fighting them.
Too simplistic, really. At the end of the day, you need a sense of enigma, which "You talk, or me hurt you" cannot deliver.
Also like the idea of a large battle being the end and this could mean your mission is to search the wastes for supporters to fight this battle.
What about my main mission to find out who I am? Maybe if I go to a big battle I'll know.
A bit like the lord of the rings narrative but in a post apocalyptic setting.
I think you'll find that LOTR's narrative was based around the destruction of the ring of power, and they chose a small band of heroes to do this task. Note small band ont amass an army. The battle at the gates of Mordor was somewhat an inpromptu idea, made to cause a diversion.