There are no teams mentioned in the report, but it does say the MLS league office is helping move things along. From a purely speculative position, I'd assume the Sounders would prefer to trade him to an Eastern Conference team. Wahl's report also says that the league seems to have accepted the idea that Johnson deserves a raise beyond whatever his option price is. Johnson has clearly earned a significant pay bump, as he's led the Sounders in scoring in each of his two seasons here and gone from a complete national team afterthought to a near lock for the 2014 World Cup team.
Still, the trade rumor is not necessarily a huge shock, either. Johnson was notably not among the players mentioned by Sounders GM Adrian Hanauer while discussing the state of the team moving forward, despite scoring more playoff goals (three) than any other player in team history.
For all of Johnson's obvious attributes, he could be commanding a Designated Player salary and the Sounders aren't expected to have a ton of wiggle room under the salary cap. He also seems like he might be surplus talent, as it was never clear that he and Obafemi Martins could form an effective strike partnership.