I've never played UU (well, maybe once in ORAS), but I'll give my thoughts on it anyways
The Grass / Fighting typing on Tsareena mostly gives it STAB on High Jump Kick, making it able to dish out a lot of damage. It can now pick up KOes where it once couldn't on mons such as Blissey, Cobalion and Empoleon. Blaze Kick, on the other hand, allows it to have coverage for the mons that previously walled it, such as Forrotress, and any uninvested scizor. Blaze Kick doesn't impact it as much as the fighting type, however. While the Fighting Type on Tsareena my look extremely strong, it now means that it is weak to Fairy and Psychic, and 4x weak to Flying. This introduces new counters to Tsareena, such as Clefable, Metagross and the latis, althought the latis are still hurt by Play Rough
+ Sand Stream
This one is great, Sand Stream not only gives passive damage to any opponent not Ground, Steel or Rock but also makes Diancie have an incredible SpD, probably this would make Diancie as good in UU as Gigalith became in RU with Sand Stream.
Both Scarf and Band with U-Turn would be awesome, Megahorn is great but Bug coverage sucks (resisted by Fairy, Fighting, Fire, Flying, Ghost, Poison and Steel), so make Hera able to hit 'n run would be great for this beetle.
+ Psychic/Dark typing and Nasty Plot
STAB in Dark Pulse would be great since this think lacks coverage, it would definately become a cool mon, but that 3x to Bug-type annoyes me.
+ Grass/Fighting and Blaze Kick
That would make Tsareena much less a utility mon, it would become a great offensive one. STAB High Jump Kick is great and I could see both Scarf and Band sets becoming nice, it have a good 120 Attack, high stats and a coverage in Blaze Kick to hit both Forretress and Scizor, it's also faster than Bisharp and Scizor, so they wouldn't wouldn't be able to revenge kill even the CB set thanks to Queenly Majesty+Higher Speed. On other hand, lose part of it's utility with weakenesses to Fairy, Flying and a 4x to Fighting would be a little bit annoying for Tsareena, still yeah...I think it's good.
252 Atk Choice Band Tsareena Blaze Kick vs. 252 HP / 252+ Def Forretress: 300-356 (84.7 - 100.5%) -- 81.3% chance to OHKO after Stealth Rock
252 Atk Choice Band Tsareena Blaze Kick vs. 248 HP / 124+ Def Scizor: 424-500 (123.6 - 145.7%) -- guaranteed OHKO
252 Atk Choice Band Tsareena High Jump Kick vs. 0 HP / 4 Def Volcanion: 171-202 (56.8 - 67.1%) -- guaranteed 2HKO
252 Atk Choice Band Tsareena Trop Kick vs. 244 HP / 8 Def Gliscor: 133-157 (37.7 - 44.6%) -- guaranteed 3HKO after Stealth Rock and Poison Heal
252 Atk Choice Band Tsareena High Jump Kick vs. 0 HP / 0 Def Cobalion: 322-380 (99.6 - 117.6%) -- 93.8% chance to OHKO
252 Atk Choice Band Tsareena Blaze Kick vs. 252 HP / 0 Def Metagross: 210-248 (57.6 - 68.1%) -- guaranteed 2HKO
252 Atk Choice Band Tsareena Trop Kick vs. 184 HP / 0 Def Blastoise-Mega: 278-330 (80.5 - 95.6%) -- 56.3% chance to OHKO after Stealth Rock
252 Atk Choice Band Tsareena Trop Kick vs. 252 HP / 80+ Def Hippowdon: 240-284 (57.1 - 67.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
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.
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
I'm going to essentially revive this. My personal favorite is mine (obviously). It gives a better defensive typing to Necrozma while increasing STAB. Also, a weakness to Bug isn't a huge deal as you only need to worry about U-Turn and would likely use a bug-resist to absorb the hit. Here is a special attacking set that would likely be common on Necrozma with my updates.
Necrozma @ Life Orb
Ability: Prismatic Armor
EVs: 252 SpAtk / 252 SpD / 4 HP
- Nasty Plot / Calm Mind
- Psychic / Stored Power
- Dark Pulse
- Power Gem
This set is made into a bulky NP sweeper which is something not really seen. Calm Mind can be run instead for more bulk. Psychic is STAB and Stored Power can be used if you can get to +6 (somehow) for a massive payout. Dark Pulse is STAB and Power Gem is coverage for Bug types.
Diancie + Sand Stream
In my early days of ORAS UU, I messed around with a sand team basically revolving around a Regirock/Steel manually setting up sand, switching into Diancie, and proceeding to wear down the team until it was in range of SandDoge. Naturally it was very bad, and I wondered how much better it would be if Diancie could set sand itself without using a moveslot.
Then I saw this Theorymon.
I think that Sand Stream Diancie would single-handedly make sand viable in UU. As G-Luke mentioned above, Diance+SandDoge make an extremely good core, as they blow apart each others checks. Its Fairy-Rock typing lets it hit every type bar Steel neutrally, and with Earth Power, it gains almost perfect neutral coverage.
I think sand will be a great addition to the meta, as weather has always been niche without Drizzl/Drought and it would give players a whole new concept to play with. Sand will probably never become overcentralizing due to the presence of Scizor, either.
Those are just my two cents about Diancie and sand in general :]
Really sorry for my extended absence, external exams just finished, so now that I have a little free time I can get back to this. Sad that so little discussion happened when I left though, just hope that the initial interest can be resparked. To get things going, I'm just gonna run into voting