If I may throw out my tidbit: Blasters are mostly treated like a rifle in that they fight their best at medium-long range, belting out damage and killing targets long before they can get close enough to do significant damage. I was thinking that Sentinel's should be treated like a shotgun: Somewhat mediocre (Essentially about 5-10% less damage than what is currently done) damage at the range limit, devastating damage at point-blank range (About par with a Stalker, depending on balancing). It becomes a balancing act: Do I stay at range and pelt my target with weaker attacks, or do I charge in and rely on damage mitigation to keep me alive while I belt out damage?
Now, separate from my little bit above: Opportunity's biggest flaw IMO is that if the target tagged dies, it doesn't allow tagging another target with the debuff while the bar is charged. What I propose is that if Opportunity is active, every shot belted out procs the debuff on all targets hit, even the AOEs and Judgements. So, if I trigger Opportunity and kill a target with it, I can switch to the next target, attack it, and it will get the debuff. If I decide to use a targeted AOE (Lets say, Rain of Fire) on a pack of enemies when Opportunity is up, all targets hit even once by it will get the debuff.
A tweak to the Opportunity bar itself: It charges with attacks as normal and activates once charged, but instead of completely draining after a set time period, it starts a slow constant drain (until empty) once full. Attacking will add time to the bar. A well built Sentinel can keep Opportunity up constantly when in combat.
As for selecting an Opportunity type for when it does proc, make it like the Adaptation selection for Bio armor or the ammo type for DP. You click it, its set, and will pop that type when the proc goes off, with a minor flat effect when the proc is not active. Ex: Offensive grants a 5% damage boost when selected but not procced, a 20% damage boost when procced. Defensive grants a minor constant heal tick (about 1 tick per second, 1% HP per tick), which gets a multiplier when procced that scales it up and down according to how much damage was dealt each heal tick (up to a maximum of 10% HP/Tick, but the scaling to get this high requires external damage buffs).
Too much? Not enough?