The new "debuffDR (dbDR)" system results in the myriad of small-magnitude debuffs in the game being eaten entirely, especially when thrown by non-debuffer classes who don't have the relevant combat skills (Opportunity, Lingering Malice, etc.) to penetrate the dbDR, and yet have inherent debuffs still tacked onto their powers. It also heavily penalizes multi-debuff powers, like Rainbow Quills, which should normally be excellent choices for debuff-focused characters.
The inherent dbDR of higher-ranked mobs is tied to a new status, say, debuffBarrier (dbB). The dbB status acts similarly to hpBuffer -- A high-ranked mob would start with a given pool of dbB -- let's say 500 for the sake of argument, although it could easily be tweaked higher or lower pending balance testing. Whenever the creature's dbDR would reduce the magnitude of an incoming status effect, the mag is also removed from the creature's dbB status. Once the creature's dbB is exhausted (offhand, I'd expect the general balance point to require around 8-10 rounds to do so, unless a concerted effort is made), its dbDR no longer functions, and it is fully vulnerable to debuffs.
Optionally, creatures of high enough rank (bosses, primes, etc.) would have a special skill added with a large charge time which allows them to replenish some fraction of dbB. It can't be too fast, or too much, or else it would just trivially refresh the barrier faster than it could be brought down. Perhaps something in the 2k-3k Charge range for 50 dbB, or 5-10k charge for around half the initial value (again, these numbers can be played with for balance purpose).
The idea is to create a point (or a window, if a dbB replenishment power is included) which becomes a "burn phase", in which the players can unleash their most potent powers for maximum effect. This would give smaller 'plink' debuffs a way to contribute, as they would still help chip away at the dbB, assisting the party's debuffer in doing so even while their smaller, incidental statuses were absorbed. It would also give big fights more interesting gameplay, as rather than being the same from beginning to end players would have a sort of gauntlet to endure during the early phases of the fight when the dbDR is still active, while getting a "payoff" if they're able to survive long enough to bring down the barrier. It would also create potentially interesting decisions during the fight -- do I use this large-mag, huge-cooldown debuff to whittle away a good chunk of the barrier, or hold it in reserve to unleash upon the boss once it's vulnerable?