Dubious at best. Fallout 3's graphics are already excellent. I would think that if Bethseda was going to remake one of their old games with modern-day graphics, it would be Morrowind, since that game is much older, but many ES fans swear that it's their fave.
But I don't think it's going to be either. We'll get Fallout 4 and ES VI instaed, which works better for me anyway. Remakes and reboots are two things we've already had way too many of.