What We Already Know:
Very little. Though Koei had a trailer of Trinity: Souls of Zill O'll running at last E3, it delivered very little information aside from the fact that it appeared vaguely fantasy-RPG-ish.
What We're Seeing Now:
Luckily, that was a pretty correct impression! Trinity is an action-RPG featuring a trio of heroes with sharply distinct abilities. One is a traditional swordsman; another is a beefy tank type, and the third is a svelte, dual-dagger-wielding lady-rogue. The differentiation between these characters is significant, because the player needs to constantly swap between them on the fly. Part of the reason for this is that some characters are simply better at certain things than others; the game has a prominent magic system, for example, but not all three are equally good at magic.
But the bigger draw to the hot-swapping style is joint attacks. Especially when fighting bosses, the characters can quickly switch off attacks, gradually building to a massive, three-pronged maneuver. It's possible this is the game's only gimmick, but if so, it at least adds a slightly new flavor to the traditional hack-n-slash RPG.