I think the Engineer class and idea is great. I've been thinking about the the primary damage skills Wither and Leech, especially since seeing several people claim Engineers need to do more damage. While I'm underwhelmed with Engineer damage, I'm not really in favor of simply adding some point blank, insta-cast AOE skills for this class. Here are my thoughts from a lvl 13 engineer:

The most efficient way to use these skills against regular mobs is to fire one off against one mob, then manually target another. That is because regular mobs die in 2 or 3 seconds using these skills and autoattack together. Wither takes 10 seconds for full damage, while Leech takes 6 seconds. A player is thus lucky to get half damage out of either skill. More problems arise when mobs are bunched together, making manual targeting difficult, and because we cannot predict which mobs our commando and operative buddies will kill. It is also a bit counterintuitive, keeping mobs ALIVE longer so you can fully damage and KILL them. Just when you finally DOT 2 mobs while you attack a third one, you can die because all 3 mobs are trying to KILL YOU too (happens to me).

That said, these skills do tend to do max damage on high hp enemies and bosses, which can sometimes live through multiple casts. Are they worth it for that purpose? Probably not. I've all but stopped using them at lvl 13. But maybe the solution isn't to just turn the engineer into a PL enchantress with L33tning and Fire Blast.

Instead, some skills, like the Operative skill Amplify Pain, have an effect better for bosses (armor debuff) and another for general play (crit buff). Could that be applied to Wither and Leech somehow? Here are some ideas:

1. Add some other effect to the skill that doesn't depend on DOT (slowed movement, root in place or a hit debuff?).
2. If the skill ends early, have its remaining duration "infect" one nearby enemy. No other enemy? No infection.
3. Add some kind of bonus if the skill finishes on a mob (a stun, knockdown or mini-pain explosion, maybe?)
4. Shorten duration and increase potency (most direct).

One of these would make the skills more useful. I'm sure there are a bazillion other ideas too. Post your thoughts!