As the saying goes, you can never have enough pitching prospects and the outlook for LHP prospects in the system has improved in 2017. I'm not sure you can look at any of the guys listed below as "definite rotation piece for the future," but most have progressed nicely this year.
Leibrandt
Irvin
Suarez
Romero
Fanti
Falter
K.Young
Mezquita and Parkinson have done ok in very limited samples since the draft, too.
Unfortunately, the only guy to fall off was probably the top LHP starting prospect coming into 2017, Elniery Garcia.
Mills is a nice name to add to this list.