@DarkHelmet
Instead of making Concentration affect Necro’s bone spells look at these proposed improvements - Improvements for necromancer
Replacing Poison Explosion, which is in fact copy of Corpse Explosion mechanic, by Vile Alchemy(or Rotting Marrow) opens other way to buff bone Necro’s output damage in more fitting to the class setting way. It’d also allow to deal with magic immunities and monsters’ regeneration, also an unique feature for a caster character.