The problem with the engineer is there is no way I can see to make him compete with our other heroes for the slot, without making him very strong. I am worried that too much bonus to the engineer, or a global bonus that he grants, make Empire gunlines too strong.
I playtested the engineer with his targeting monocle and the +1 on misfire rolls. I really like the +1 to misfire mechanic, it goes great on a Hellblaster, although he only got to shoot his Long rifle with BS 5 once.
The problem was, as Fandir pointed out, that he competes too much with our other heroes for important combat, Ld, and magic slots. Even in a Gunline, a mage is usually a better choice.
However, I found that with the simple inclusion of an Engineer and General of the Empire, our special slots were wonderfully free: I could take a unit of Greatswords and a Mortar, and still have space for 2 cannons and two more choices! It was, perhaps, a little too strong, and felt a little too much like High Elves. I think any special and rare choices that get moved to Core because of army structure will have to not count against the minimum number of core units.