I wouldn't get my hopes up for this game living up to the name Fallout 3 simply by virtue of how I perceive the modern game market. Fallout 3, to live up to the name, would require either a repetition of certain shared characteristics of Fallout 1 and Fallout 2 or a reasonable evolution of those very same characteristics. This includes the setting, atmosphere, and mechanics. Setting is the post-apocalyptic world and I believe that is very much accomplish able. Atmosphere is things like the humor and the mood the game evokes beyond the setting. Mechanics is the stat system and the turn based combat. It is mechanics I think the problem will be present. I have doubts that the game market, as it exists today, would be sufficiently receptive to a turn based RPG as to make that game a success. I also doubt that there is a reasonable evolution of the mechanics possible that would be sufficiently recieved. And I also think Bethesda likely agrees. Of course, this is only my opinion of the market and an assumption about Bethesda. Ultimately it would only come down to whether Bethesda agrees or not.
With just the setting and atmosphere, would Fallout 3 fit my criteria of a successor? No. Would it fit my criteria of a spin-off? Most definitely. Wouldn't even need the humor in that case.
Is it likely to be an awesome game? Damned if I know, I can only hope that regardless of whether or not it has my three criteria of a successor that it is fun.