The engine I think should be used, should be the same one that Fallout 3 is using, nobody plays a game that build with crappy stuff anymore I think , however instead of just all action this time how bout make the characters with more story behind them? Give more characteristic to the characters. To do so I have a whole new idea, how bout 7% adventure game/ and 93% the usual fallout bloody mess. For example instead of just needing 2 Action points to reload, this time you have to have your ammo ready within your firearm to fire, you open your inventory(pocket) on the right, with the character holding his let say revolver on the left, you have to drag each round into each revolver slot, as for pistol with a mag you have to drag each pieces of ammo(bullet) onto the top of the mag, and see your character's hand putting the ammo(bullet) into the mag. (This is one example of how to make it more adventure game-like)
This time the setting, for the god sake no more future of random shooting, random characters that doesn't make the plot of Fallout any stronger. The setting how about take place in the pentagon? how about two weapon development staffs in pentagon as the protagonists, Also how about they are lovers?(come on it makes the story way more meaningful than fallout 2 where the protagonist can have like 100000 lovers, it destroys any romance in the story) I still have more idea on this(the storyline for fallout4) but I want to know what other fellow fallout fan think so far?