With 300 ms real time combat system makes no sense , so how about a turnbased combat (stolen from another sandbox mmo ofc)?
For example: Let's say we have a guy with big , two-handed axe against a guy with knife and shield. When they enter combat both have a 3-5 sec meter , depending on weapon - slow big axe and fast knife. While knife will do 1 damage every 3 secs , big axe does 2 damage every 5 sec , so knife does 20 dmg per min and axe does 24 dmg per min , BUT guy with knife got more armor because of shield.
And to make it even more complicated another meter with action points for combat skills , which can be used instead auto-attack each attack timer tick. Skills differ for each type of weapon (+shield) , to make soething out of different weapon - knifes for fast attacks light attacks , clubs for slow but heavy damage , axes for something between. Also if anyday they'll add staves and magic that would leave some place to balance them.
And last thing - ranged combat. They'll fit easily in turn-based formula - n sec to pick an arrow , take aim and shoot.
So um , cons of that system except maybe a little less "real world" feel (yeah , much better than teleporting hobo with axe ganking while you stop to catch breath)?