(Read the last entry if you don't know what I'm talking about.)
After suggesting a Phione-Manaphy sort of relationship for these two (the difference being Carbink can be found in normal gameplay), I took a look at Phione's and Manaphy's movepools to see what we could expect for Diancie. And... well... it's not pretty. Looking at the level-up lists, they're both identical, except for Manaphy getting Tail Glow, a quasi-signature move, and Heart Swap, a full-fledged signature move. The rest is all extra TMs. This means that barring a surprise, I'm expecting any move that's not a TM to be off-limits for Diancie. Since Diamond Storm is a physical move, the worst part is no Play Rough to complete its physical dual STAB - you'll have to make due with Moonblast if you want a Fairy move. Heck, it's not like it's all that much better in Carbink's offensive movepool otherwise - there's Power Gem, Return, Psychic, and that's about it.
Do you see the problem now? Diancie's meant to be exactly identical to Carbink, aside the fact that it can actually attack. And with such a terrible offensive movepool, let's just say it's not going to be a whole lot of an improvement. Hopefully Gamefreak took notice of that and gave Diancie a few extra TMs. Like Earthquake. And yes, I know with that stat spread Diancie's still going to be all about the defense anyway, with dual screens, Stealth Rock, Trick Room and other such moves. I'm just saying if Carbink is this bad, and Diancie retains the same terrible movepool bar Diamond Storm, extra attack isn't going to do jack. And that's just terrible.