For Buffalo I would guess Kane - O'Reilly - Ennis and then Moulson - Girgensons - McGinn. I don't think they give Eichel top-six minutes out of the gate. There's no need for this team to do so. Also, Gionta is no longer a top-six forward, and would seemingly make a good "mentor" line mate for Eichel (for whatever that's actually worth).
I can't imagine Washington doesn't at least try out Ovechkin-Backstrom-Oshie to start. That would seemingly make the second like MaJo-Kuzya-Williams.
I don't really see either team as really having a lot of options. Guys like Beagle and Wilson for Washington aren't top-six forwards, and Burakovsky is behind Kuzya at this point. The only change I can see for Buffalo is Eichel ahead of Girgensons, but again, there's no need to expose him to tough competition next year, at least until he proves he can handle 17 minutes a night. .
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It's Karlsson and then everyone else.
The next four after I see as some mix of Burns, Subban, Letang, and Giordano. That assumes Letang is healthy, of course. Hedman, Carlson, and Shattenkirk are all candidates to be in that mix, too.