My Engineer is level 16 now and I thought about what you said. The class & build for my main character must be both fun to play and useful in organized warbands. Since I'd like my Engineer to deal good single target damage, I think I will keep is as an alt for situations where AoE damage and the Electromagnet ability is not required. I'll make it a scavenger too. This is the build I have in mind for it:
RoR.builders - Engineer
Playing the Slayer has been a lot of fun. I set mine up for cultivating and apothecary, feeling this could be a main.
My Runepriest is level 16 too and is still interesting.