While It would be good for those who like those kinds of games, it seems most people were introduced to the franchise with 3, NV and 4, so it'll probably never happen. Of course, If it does, I'd figure it would be a free game like Fallout: Shelter, and have less time put into it than the earlier games.
If they are going to make any more spin off titles, they're going to do something similar to New Vegas where they hand the reins over to different devs while Beth focuses on the next game, although I have a feeling it won't be Obsidian again.