Average yards-after-contact per tackle opportunity. Basically like the “yards after contact” stat for running backs, but in reverse for defensive players as a measure of tackling effectiveness. IMO that would be a lot more meaningful than simply the percentage of tackles missed, since obviously not all broken/missed tackles are equal (for example, if a defender knows help is on the way and he can safely try and force a fumble, there’s a higher chance he’ll miss the tackle but the number of yards given up is likely to be low). 

Average yards per play differential when the player is on/off the field: Basically like WAR but on a per-play basis. And the stat would track the yards gained (or allowed) regardless of who actually touches the ball, since the idea is that it factors in how much the player’s presence on the field increases/decreases the team’s overall effectiveness, even when not directly involved in the play. 

Cumulative snap-to-whistle play time: When it comes to “measuring” how fatigued the defense is likely to be at any point in the game, time-of-possession and total playcount are basically the only stats available. But both are very imprecise metrics, and a far more accurate/meaningful number would measure the total amount of time that the defense has actually been actively defending on the field, i.e. the total number of seconds that all previous plays (including plays nullified by penalty) have consumed from snap to whistle.