Niskanen totally isn't getting traded this year, but Barzal, almost 21 years old, has 16 goals and 37 points in 53 games, with respectable possession numbers.
https://www.hockey-reference.com/players/b/barzama01.html
That's a heck of an offer, and if rumors are true that Green really wants to head back to Washington, and Barzal is seriously offered, yeah, I think Niskanen just became available. I mean, we're still a 2nd round pick apart, but... wow.
The conventional wisdom is that the Caps don't trade for Green because he'll cost a 1st round pick, and the Caps need one of those to spend on a forward, at least every five years or so, but Barzal pretty well fills that need, to say nothing of Connolly's breakout season.
Now I just can't wait to see what Edmonton fans think about going through another summer of ridiculous free agent negotiations. I'm betting they'd be happier if you wanted Drai and picks.