If you are talking about player character Melee AT's, then this is already how it works. Resisting Mez and Resisting Debuffs are two different things, and many Mezzes (at higher levels) already come packaged with Debuffs. For example, the Rikti Magus Block of Ice power is a Hold and a Slow,. Another is the Earthquake from a Ruin Mage is -Defense. These things have to be resisted separately from the mezzes, and not every Armor set available to Melee AT's has the appropriate debuff resistance.
If you are talking about giving Dominator Mezzes additional unresistable debuffs, then... well that sounds so "awesome", I suspect it wouldn't be implemented.