If we go all the way back, like reeeeally far back, when the Berserkers were still the Lions and my first player was playing for them, one potential original rival could've been the Outlaws. They were both expansion teams from the same season. However, not only the Lions have moved, so have the Outlaws and they're now known as the Yukon Malamutes. So with both teams not having much to do with the original teams anymore, that rivalry is probably dead, at least I couldn't really find a reason why Newfoundland and Yukon should be rivals. A current rival could potentially be Maine given the playoff history these two teams have had in the late 50s and early 60s. At least I wouldn't be surprised if Maine saw Newfoundland as their big rival after losing so much, all the time. Other than that I have no idea who could be a bigger rival so my pick is Maine.