Hi, I'm still kinda new to NatDex, but I searched around and didn't find any commentary on Drei, at least not after Spectrier was banned (checking Trier might have been Drei's biggest role in NatDex to date).
Thus I think answering your question may be not easy for anyone to reply, and so I'm giving it a shot.
Something to note if you haven't already, is that it's ranked B+, so just know it has some flaws when compared to Tyranitar or Ash-Greninja as Dark types, or when compared to Mega Latias or Garchomp for the resists that Dragon provides.
Seeing that Blissey and Teleport could really put Drei in a bad spot, then you might want to consider Superpower to at least punish and limit Blissey before it ports out. Hits Heatran decently and hits Tyranitar hard too.
If I had to use NP Drei now, then I'd try out an Expert Belt set like this:
https://pokepast.es/46ee1e4727484881
I feel when you only have 2 special attacks and you will be planning on using NP in the late game, then Expert Belt is better than Life Orb since I think the recoil without roost can be very detrimental.
Hasty since taking Magma Storms is one of it's main roles.