Although I love the idea of getting Nylander, The blues main defensive core is very important because our goaltending is weak. And personally, I don't think that he is very important for the Blues to get (after all of our off season moves).
I personally think that a trade that would benefit the blues more would be:
Steen Rights to Nylander
Boumeester for 7th round pick
4th round pick.

This would give Toronto some goal scoring and some much needed defence. And St. Louis would get Nylander and a Salary dump