Or have it so that it only focuses on the first one that got close with a tower shield but then that one just keeps on moving around while others attack it with impunity.
So you would have to build in loads of conditions, like some one with a towershield gets first aggro but when someone does more than x damage aggro changes to the one doing the damage.
There would always be ways to exploit such a thing, everyone has a big damage weapon and other small damage weapons and you keep on switching between the two to juggle aggro.
How it is now with the one that is closest gets it is just straightforward.





