A lot can happen with a rookie rb's in one off season and with more competent play from an O Line etc etc.
Le'Von Bell had 244 attempts for 860 yards for a 3.5 ypc as a rookie and everyone was writing him off and saying it was a wasted pick and a bust.
Hard to see how improved blocking wouldn't help.
If we are specifically comparing with the Bell situation, and probably you didn't intend the situational comp this literally, but just for the record - he lost a lot of weight and looked like a completely different player in his soph year. There is no similar material reason or identifiable cause/mechanism of change in the case of Sankey, to offer parallel hope of improvement, at least from that direction.
But maybe it will be something else (like blocking, which many seem to be pinning there hopes on).
* While on the subject of blocking and free agent prospects for improvement, not sure how RT Barksdale fared in his run blocking grades (would almost have to be better), but PFF had him as allowing the fourth most pressure in pass pro among OTs in '14, where I'm pretty sure he slumped relative to '13. He was in a group that included LT Kalil of MIN and rookie first round RT James of MIA. There may be a reason no other teams seem to be showing interest besides STL and TEN, their's seems to be tepid, and he remains unsigned weeks after the beginning of free agency. Which probably wouldn't be the case if he was more widely viewed as skilled in either run or pass blocking. Maybe there is just way to much of a gap and disconnect between what he wants and others are willing to pay, and/or guaranteed money, length of contract parameters, etc., and the rest of the league knows it (anybody could call his agent, and maybe has, but balked at the price, rendering a visit pointless, until budging on it, or being more flexible on other potential contract demands)?