Hey all, in a 2 man campaign where a warlock is the only spell slinger I was thinking of allowing an item that would permit modifying the spell damage of Eldritch blast from simply force to include some of the elemental damages.  The warlock is celestial so I know adding the option of fire would permit more power, but would other elements be too unbalancing?  Also if I go through with it does anyone know how to do this?  When I looked at changing up an item type, it allowed changing the range of eldritch blast, but I couldn't see any where to change the damage type.   

 If you mean reporting the changes to Eldritch Blast in DDB, there's no way to do it programmatically.  You'd have to create custom attacks in your character sheet for each variation of Eldritch Blast you use, if you wanted to track them.

Force damage is already one of the most powerful/useful damage types.  Very little resists it, and it cuts through numerous other forms of resistance.  Changing Eldritch Blast's damage type, generally, is either a wash or makes the spell weaker.  The only exception is when attacking enemies with specific vulnerabilities, at which point if the caster can choose damage types freely it's quite a powerful benefit.  Recommend making him decide what damage type he does on short rest, rather than whenever, if you want to experiment with this.

Force damage is the least resisted damage in the game. Only 1 creature is immune to force, none resist it. And considering all the invocations that already buff It, it is the most powerful cantrip and one of the more powerful spells. It does not need a buff.

Plus celestial warlock has sacred flame for radiant damage.

Thing is if the character multiclassses into draconic sorc the elemental damages can be improved with Elemental Affinity and that would stack with Agonising Blast. It would be fine if you declared that changing damage type to another elemental basic like fire, cold, etc will invalidate Agonising Blast. Then it's not overpowered.