Thor is only there for extra damage and will most likely be replaced by any other thunder SSR you get in the future. She will improve your team in only the fact she hits hard but she will not be big factor towards clearing rank 4 accessory quests outside of water. In fact, thunder teams are the worst choice to do this since they do not hve the heals to do so, unless you use Sol.
Amaterasu for Fire
RyuOh for Water
Raphael for Light
Satan for Dark
I don't reccomend either dark or light, until we get more SSRs.