I'm not sure that I'd favor a hard edged mech marker. It suggests that it might be the hitbox and seems likely to cause confusion in newer players. That being said, a circular health bar would be nice, especially with segments. That might suit the purpose without making it seem like a hitbox. An alpha'd color cloud (like Warcraft 3 hero colors) might also work well.
Familiars: Yes, yes, and yes.
Modules: The more restrictive the additional shot, the more interesting. Examples: Added grenade launcher (bounces and explodes explodes), mine launcher, and such. If it just plain adds damage, you forget about it. Auto-target generally just adds damage, as do boring sprays/shots that just go out and do nothing special. If it is going to be a simple damage add, something interesting like "Emulator" that doubles your normal shot actually more interesting (ignoring the exact dps) than a static new bullet. Especially if the two shots both shoot at +/- 5 degrees so you no longer have a straight ahead attack.