Warlocks really need a way to remove elemental immunity. This may have not been the case with the original pacts as the spells are more CC and instakill orientated, but the newer pacts like Abyss and Carceri Storm focus so much on negative energy damage and cold damage that they become quite useless against a foe that is immune to those types of damage.
Adding an ability to remove the immunity from enemies would make a huge difference to support these play styles. This could be added as a lvl 12 feat.

When it comes to Abyss it would make sense to have them benefit from their own aura for healing purposes.