Alternate start mod are pretty popular mods with Bethesda games and I'm pretty sure fo4 will have it too. This time however there is a slight problem, the heavily railroaded intro of fo4. How can you have an alternate start where the intro makes sense? Being a random mercenary, make a family with nate/nora, get teleported back in time, live in the 70s, get frozen...

How about the main quest is skipped entirely but still have the main quest progress? The player isn't the sole survivor but is instead settler #3852. Sole Survivor would be made into an npc and can be randomly encountered in the common doing random stuff (eg when you goto concord you could SS ramboing in power armor or returning to diamond city with nick etc.)

upsides

+don't have to deal with time travelling

+don't have to voice the new player character (basically be like good* ol' days). This also could help out user made mods which try to include new player dialogue.

downsides

-lots of smaller things won't make much sense (ie companions, quests where SS's background is referenced (USS voyage)) which might need to be locked from the player

-setting up random encounters with SS

*goodiness depends on the tint of your glasses.