+ Psychic/Dark and Nasty Plot
While the crippling U-Turn weakness may hold it back, the new Typing is generally an improvement, and plays on Necro's offensive capabilities. Automize sets, Offensive SR and Calm Mind sets are all improved, and Nasty Plot is the icing on the cake.
+ U-Turn
Heracross is usually outclassed as a Scarfer in UU due to it commonly locking into moves that can sap its momentum and its speed. While its speed is still an issue, U-Turn allows to click a strong attack thanks to STAB and preserve momentum at the same time. I think its a good look for Heracross and would make it more viable.
+ Grass/Fighting + Blaze Kick
This probably lets it solidify its ability to be both a good defensive and offensive Rapid Spinner, and have a decent niche in Choice Band. I personally don't like Scarf on Tsareena, as it requires too much support to work to me, but its new resistance to SR at least lets it click U-Utrn more often and spin easier.
+ Sand Stream
I love this. Sand Stream Diancie alone allows Sand Cores to be more than just a sad gimmick in UU, as alot of things that threaten SandDoge are blasted by Diancie, while providing Cleric support. As for Diancie itself, it allows the Diamond to outlast opponents with Chip damage and cranks up special bulk to an eleventh.
252 SpA Choice Specs Primarina Hydro Pump vs. 252 HP / 220+ SpD Diancie: 290-344 (95.3 - 113.1%) -- 75% chance to OHKO
252 SpA Choice Specs Primarina Hydro Pump vs. 252 HP / 220+ SpD Diancie in Sand: 194-230 (63.8 - 75.6%) -- guaranteed 2HKO after Leftovers recovery
+1 252 SpA Raikou Thunderbolt vs. 252 HP / 220+ SpD Diancie: 111-132 (36.5 - 43.4%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
+1 252 SpA Raikou Thunderbolt vs. 252 HP / 220+ SpD Diancie in Sand: 75-88 (24.6 - 28.9%) -- 55.9% chance to 4HKO after Stealth Rock and Leftovers recovery
Looks good.