I think a cover system is better without an accuracy mechanic, because the randomization makes a game less tactical. Cover could reduce the damage by 50 %. A flanking mechanic makes a game much more tactical and it should be as important as the cover system. Flanking could increase the damage by 50 %.
A range mechanic would be great, too. Every weapon could have an optimal range and if the target is not in that range the attack inflicts less damage. For example a shotgun could have an optimal range of 5 tiles and if the target is 12 tiles away the damage could be reduced by 70 %.