I just started diving into this and my immediate thought is FB, because that's the only position I don't have 2 99s at now, but I read here that the fullback with all upgrades isn't worth it.
Next thought would be something like one of the tall CBs, but I already have 5 99 CBs running.
Punter or kicker might make sense as well given there are no 99 options for them, but I never punt and rarely kick. Which players have you done?