Matt Tkachuk just signed for 8 Yeats X 9.5M after finishing 6th in league scoring and also bringing a lot more physicality than Nylander.
Gaudreau just signed for 7 years X 9.75M after finishing 4th in league scoring (not his first top 5 finish).
Filip Forsberg is in the first year of an 8 years X 8.5M deal.
Using those as comparisons, I'd say Nylander should be something like 8 years X 9M. He's probably a similar calibre player to Forsberg (different styles though), and not as good as the other two.
In saying that, our own internal cap structure will work against us, and he's going to look at what Tavares, Matthews, and Marner all get paid.