Honestly if stage 20 isn't too hard and stage 19 is the dolomites tappone we all are hoping for, I don't see much of an issue with Piancavallo. It's not hard enough to flip around the GC, so it doesn't stop racing like Lussari or Zoncolan would. Yet it still gives riders the opportunity to attack on a climb where in the past attacks haven't been seen that often. If they go twice Piancavallo or with La Crosetta it still would the opportunity for a desperation move to gain 2 minutes for a spot on the podium. So I rather have this:
19: 5000-5500 D+ to Piani di Pezzè
20: 180 km 4000 D+ to Piancavallo 
Instead of the other way around. I fear more that stage 19 could block action on a potential great classics parcours on stage 18.