It is a pain in the ### to find viable starting RBs in 12-14 mandatory 2RB leagues, so I'm always on the lookout for cheap players who might become serviceable starters. Some of the more interesting options out there right now IMO:
Toby Gerhart - Former 2nd round pick who actually played well in 2011 with Peterson out. There is very little flash to his game, but he's a decent back and he reportedly
drew trade interest earlier this season. The best thing about him is that he's a free agent after this season and DIRT cheap. I don't expect anyone to hand him a starting job on a plate, but there are MANY teams he could sign with and at least threaten to earn consistent touches in a committee.
Shonn Greene - 2-time 1000+ yard rusher on less-than-dynamic Jets teams. I don't think he's as bad as everyone thinks he is. If you're a believer in using objective NFL data points to evaluate players, you should make note of the relatively large contract Tennessee committed to Greene. I admittedly don't know the particulars of what it would mean financially for the Titans to get rid of Chris Johnson, but it seems possible that Greene could get 1-2 seasons there as a starter. The upside is VERY limited, but in larger 2RB leagues anyone who gets touches has value.
Mark Ingram - My level of optimism is very VERY low on this player, but we all know about his background and draft hype. If I could get him as a throw-in or for the price of a late 2nd round rookie pick, I might take a punt on him. There's some chance that he could become a Benson or Moreno on a new team. It's a low upside proposition and I wouldn't give up much to get him, but we've seen this movie before with other initial draft flops.
Jonathan Stewart - It has always been a question of opportunity and health, not ability. IF he comes back to 100% and stays reasonably healthy, there is plenty of upside here over his current cost. It remains to be seen if he can get back to his old level, but the idea that he's toast seems premature. He's younger than Jamaal Charles, Arian Foster, Marshawn Lynch, Reggie Bush, Matt Forte, and Adrian Peterson (all of whom are viewed as top 10-15 dynasty backs).
Apart from those guys, there's an interesting group of young backs who have flashed legitimate NFL talent and/or were high draft picks with compelling potential. I'd put Bernard Pierce and Christine Michael at the head of that list with David Wilson, Bryce Brown, and Ben Tate also in the picture. I don't want to get too carried away with the Michael hype because he's not a flawless prospect or a slam dunk lock, but when you weigh his youth/talent potential he should be rated much higher on generic dynasty lists. I feel the same way about Pierce, who clearly has NFL starter talent, but just needs to stay healthy and get his chance.