Just to suggest two simple tweaks to Aegis of Fate.
1. Reactivates/regenerates every X seconds, instead of % to proc when hit (scales, interval decreases as skill level increases)
2. Higher % of physical armor contributed towards Aegis of Fate HP (scales, % increases as player level increases)
The on hit condition of Aegis of Fate was always iffy, especially since it doesn't stack with Force Field; you had to get hit and lose HP in order to have that (low) 15% chance to proc Aegis of Fate. Increasing the % chance to proc would make Aegis of Fate highly similar to a % increase to physical armor (except with insaaaaaane % numbers, and stacking multiplicative).
Therefore, a change to the mechanics of Aegis of Fate was logical. Having it refresh/proc at intervals plays into a 'tanking' style that the Engineer is designed around; basically pesudo "HP" regeneration.
The reason why physical armor scales with player level rather than skill level is because monster damage scales with area/player levels. Basically prevents Aegis of Fate from being ridiculously OP at lower levels, while being viable at higher levels.
Aegis of Fate was always a cool skill for the Engineer; just sucked way too much in comparison to Force Field