Love the Expert Sphere Grid. I didn't even do anything too fancy, I just put Wakka on Auron's grid, Auron on Tidus' grid and Tidus on Wakka's grid, and had Yuna and Lulu swap periodically (their paths intersect quite a few times). Wakka ended up being hilariously powerful, Attack Reels almost one-shotted every storyline boss. It was quite fun, overall.