He's a Boras client, so probably a non starter. The Nats are already pushing up against the luxury tax for the next 2-3 years at least, and that seems to be the ownership imposed limit on spending. Extending him now would give him a much higher annual salary for the next 2-3 years and might not be worth it to get a couple of Free agent years. Having cheap, valuable, team controlled guys on your team is really key to fielding a competitive team every year.
I think you put something like Arcuna’s deal in front of him, and it might be difficult to say no, boras may advise against it, but that’s a lot of guaranteed money when he’s still a couple of years away from arbitration