how about change cap system a bit, make it works like DPA, so that anyone can attack capped mob but when they fights it their a/d capped to cap lvl of mob
example:
let's say you're 150 a/d char and fights 3 opponents: ft(cap: 100, a/d: 120), yeti(cap: 120, a/d: 125), giant(cap: 130, a/d: 145).
when yeti hits you your def capped to (calculated as) 120, when you hit giant your attack capped at 130
would bring alot of challenge, variations, strategies, invasions for all