Marrow isn't a bad hero, but he isn't really good either. I like his abilities and most definitely his passives, but here is how I would make him a little better:
I would actually make him a monster caster healer. I would keep the lure ability how it is, but I would change his 3rd and 4th ability. His 3rd ability (the 1st ancension one) I would change into an aoe heal while he also does that shocking thing. His fourth I would change to where he heals while giving full energy and boost crit. I would also boost his health.