Will V13 be M or T rated? While of course the final choice is up to the ESRB, I'm sure the devs have already decided what their target rating is, and will build towards that as an ideal (changing things if the ESRB rates it different than they wanted)
While a T rating would purpose the greatest number of players as well as keeping with the MMO norm. A M rating would be more true the series roots (FO1/FO2 were both mature titles, as were the spin-offs TES:FO3, FOTactics, and *shudder* BoS). Going for M would be a smart choice artistically and spiritually. However going for a T rating would be better Financially.
While I know that a LARGE amount of M rated games are played by those who should be playing T rated, you are still supposed to assume they aren't going to.
I believe statistically M rated games sell better than T/E, however I'm not certain of that and I don't believe there has ever been an M rated MMO so that would certainly be interesting.