Assuming Jon will be moving on from Lynch, there are a couple of "long-term" - type guys in FA in Dion Lewis and Isaiah Crowell.
I really like Crowell's YPC and his toughness. He has played in all 16 games in every season since he was drafted. He's young and has a TON of gas in the tank. He's bigger and better than either Richard or Washington. He has feature back potential for sure. The Browns never used him like they should have, so he doesn't have the stats to break the bank (I have to say, Hue is disappointing me in CLE). His highlights show he's not going to outrun CBs but he's pretty smart setting guys up to make a move to get by them.
Can't say the same about Lewis. At his size, he's a specialist and we already have very similar specialists on the roster.
Carlos Hyde deserves a hard look as well. Dude's huge. Last year he had 8 rushing TDs but dipped below 4 ypc for the first time in his career. Also, he played in all 16 games last year for the first time in his career.
Bottom line: None of these guys is really a silver bullet, unless one of them can step their game up from what they've shown so far. Crowell would need to be able to take on another 50 carries or so. But if we wanted a serious upgrade we'd need a draft pick or a trade.