Welcome back Mine!
Never thought of it as a return of the gem :0 Though I somehow doubt Inferno Overdrive will do more than firegem eruption,or hit more than one target :L
Z-moves are definitely going to shake things up.Rotom-Wash aside, anything that can learn sunny day (LOADS of things) can now threaten ferrothorn,and how defensive it is will matter less due to the shear power of the z move ,potentially in sunny weather (it always bothered me how this thing could stomach non-STAB flamethrowers).Users of Garchomp,Landorus and others that survive 4* supereffective hits by virtue of their stats will have to tread more carefully when using them.
Normals can now use Z as a stronger form of the coverage that they have already, which would effectively give them STAB for a turn.It could mean that some normal types suddenly become a whole lot more attractive(Resting snorlax could choose to run a psychic z stone to punish fighting types,Porygon-Z has more coverage,or stronger coverage,Sawsbuck running charm to remove dragons or fighters that threaten it,Cincinno's access to high base speed could allow it to be a reliable zmove user for many types,etc.).
Finally,3 hit kills become 2 hit kills if a surprise z attack is used,so wallbreaking would be easier if it's used wisely.
So much shakeup for one little mechanic!Sylveon might be super dangerous with normal Z,however anybody could get a poison attack for fairies (and grass) if they run toxic now :L