Meyer and Turnbell would both be good, but I think players have to be signed for a full year before they're traded and they were both deadline guys. They could get them as PTBNLs in mid-August though. Risky to have a centerpiece be "named later", though it did work with Pomeranz to COL last deadline, let alone both. Purke and Goodwin too. Sammy Solis, another top 10 prospect, is out for the year after TJ surgery. Lombo Jr was allegedly the deal-breaker last year..
If I trade Span to the Nats, the guys I'm looking at are Alex Meyer 6' 9" RHP with mid 90's fastball, Kylin Turnbull 6' 5" LHP with mid 90's fastball and baby Lombo, a switch-hitting IF (mostly 2B) batting over .300 for Washington (and son of Lombo Sr. of the '87 champion Twins :D.) I'd take any 2 of 3 for Span, plus a throw-in lower level guy. Span is in his prime, having a great year, and reasonably priced.