Here's the thing. You mentioned that if a playoff caliber team needed a RB to put them into the elite category . . . but there is a limited market for that when you consider:
1) How many teams are truly playoff caliber? While no one is in danger of being eliminated just yet, BUF, NYJ, CLE, HOU, IND, OAK, NYG, DET, SFO, ARI aren't looking that great at the moment. That's 10 teams.
2) How many teams need a running back or have already invested in the RB spot? NE, CIN, JAX, KC, LAC, DAL, CHI, MIN, NOS, CAR, ATL, LAR aren't exactly looking to add a RB. That's another 12 teams.
3) There are teams it doesn't really make sense to trade Bell to. Within the division or to AFC competitors. No point in making the competition better. So you can likely add BAL, TEN, and DEN to the list. I am tempted to add PHI here, as the last thing PIT wants is the improve their local competition and draw attention to another local team. That's 4 more teams and brings the total I've listed to 26 teams. That only leaves 6 teams.
4) The acquiring team has to have the cap space to add Bell (or make roster moves to be able to obtain him). The leftover teams not on the list are GB, MIA, IND, WAS, SEA, and TBB. The only teams that stands out as one that is "one elite RB away" is GB. I looked at the salary cap situation for those teams. In terms of cap space, IND has enough rook to take on Bell 4 times over. MIA had $9.7M, WAS $9.4, SEA $5.6, TB $4.9M, and GB only $2.9M.
So based on all that, it looks like GB would be the target trade partner if they wanted to take on Bell. But they also know there are not a lot of teams in consideration, so I don't think they would jump at the chance to toss in a big offer.
Bell aside, the PIT defense still ranks 30th in yards allowed and 26th in points allowed. Not sure that moving Bell would help improve the defense (unless they somehow made another trade to bring in an impact defender).