Let the Opportunity Bar build. When its full, the sentinel can use it to create a buff that lasts for x seconds. Kind of like Domination for dominators. It should not be affected by global recharge. Perhaps 30 second buff with 90 second downtime. So up 1/3 of the time if there is active building.
The Opportunity Bar should be ability-agnostic, that means any primary set attack builds it, and its use is not tied to the T1 or T2 attacks.
The buff should be group wide (so to enhance their appeal in groups) and will scale with the number of group members (up to 8). Everyone affected (perhaps up to 48 targets, so a raid) gains the following:
Exactly what the buff should do (boosts to damage, end reduction, heal procs, etc, I leave to others. But it should to at least one offensive thing and at least one defensive thing).