Realistically, I think Jesse Winker makes the most sense as the return for Wong, as they're both relatively high-priced veterans in the final season of their respective contracts. Winker had a down year this year, but he's still only 29 and OPSed well above .900 in the previous two seasons with a stellar walk rate. That's the sort of profile we need in this lineup, and Winker would have himself an open spot at DH.
Of course, Flexen would be great, too, as much-needed rotation depth, and would allow us to potentially consider trading a guy like Eric Lauer.
I just don't see Kelenic happening as he's still too talented for the Mariners to give up for one year of Kolten Wong. We'd probably have to add a prospect or two for the Mariners to consider.