Basically, here are the general concept:
The main concept of engineers is that they are able to perform a varied number of skills in their masteries at high efficiency but each of their skills has limited usage similar to res signets (however, instead of 1 time use, this number maybe increased to say...5)
Equipment Storage (Main attribute)
-This is the primary attribute for engineers and the level increases the quantity of usage for engineer skills.
Gunpowder Mastery
-"Equip Gunpowder" skill needed to use the other skills
-This mastery is quite similar to rangers' single target skills except that the skills within this line has higher damage and effect duration. Skill examples include flash bang (blinding effect), Leg Shot (ranger's pindown, head shot (ranger's concussion shot)
Catapult Mastery
-"Equip Catapult" skill compulsory
-Basically this mastery gives AOE effects in contrast to gunpowder line. This line offers effects similar to those shown in pve as well as other unique skills that gives AOE conditions, knockdown and etc.
Medicinal Mastery
-"Equip Medicine" skill compulsory
-Overall, this mastery offer limited usage skills that give very effective heal or protect effects. Skill examples includee Vaccine (which protect target from conditions for a duration) etc
To sum up the class:
This class gives effective but limited usage of skills. Engineers, played by experienced players with good timing and battle awareness, can be quite effective. But if used by newcomers, they can be risky as their resources are limited.
The Equip Gunpowder/Catapult/Medicine skills work similarly to Charm Animal, which prevents engineers from becoming overly effective.
I did not state the number of usage per skills because I do not know the exact number for them to be balanced. Hence, this is just a general concept of how my ideal engineer class may look like

EDIT:
My aim is to present engineers as a very tactical class, hence, the ideas I presented is the closest to my objective so far but they seem to be redundant in their role. Perhaps some of you may help to give novel ideas that can improve on this template, which helps prevent redundancy!