Geomancy Diancie would be interesting. Without investment, it's not outspeeding anything even at +2, but with investment, it could outspeed base 130s with a neutral nature and almost match base 150s with a Timid nature. Of course, with its 50 base HP and 100 base offensive stats not backed up by any boosting item, it wouldn't have an easy time justifying Speed investment... Of course, with its defenses and the Geomancy boost, it could just go max HP/SpA and plan to tank whatever hits while smashing things. Still, especially with Diancie's bad coverage it seems like it'd have such a risk of getting stopped and forced out one way or another, and Geomancy is a one-shot strategy. As powerful as the move is, it's risky enough that I don't think it'd instantly make Diancie unbelievably broken like with Xerneas. Or anything offensive with the move, really. Just imagine Geomancy Latios, or even Heatran.
Of course, using Geomancy and Oblivion Wing together would be a pretty fun combination if it gets both, since it'd have the power to hit hard and the defenses to use the stolen HP, and its stat distribution means it'd get a lot more out of the 75% HP drain than others would. Oblivion Wing could be pretty fun in general for that reason, as even aside from the effect, it's a valuable 80 BP coverage move.