Since Lightning got a shield, I think Serah should have also got a shield of her own (aside from the small one she has in her Style and Steel outfit), and a bowsword that did not require Mog to transform into it, allowing him to act as a party member rather than a weapon.

Also, I think it would be really cool if Lightning had a sword that combines elements of her Blazefire Saber, Serah's bowsword, and Squall's gunblade in VIII, all in one. She can fire both gun bullets and arrows (each will serve different purposes), and can also vibrate the blade for quicker and more powerful hits.