I'm looking to build a Roy Mustang (from FMA) style character, whose primary method attack is a fiery explosion. While there are many Fire-based spells and cantrips I can use, I'm focused on the interaction of the Kiss of Mephistopheles eldritch invocation (see Unearthed Arcana: Warlock & Wizard) and the eldritch blast cantrip. For this character, it would make more sense for him to launch a flame, rather than a beam of force, that explodes into a fireball.
Is there a RAW way to adjust the damage type of eldritch blast to fire? I would be happy with a partial damage split, as long as there is fire damage being dealt.
He has to be Warlock 5, but multi-classing is allowed
We are allowed to use the official material, plus UA material (after running it by the GM)
Magic items will work, but they will need to be somewhat easy to get ahold of.
Note: The GM would likely be fine with allowing me to change the damage type, but I'd prefer using existing content that allows this instead. There have been cases of "You changed a rule for them, so you should change a rule for me" and I want to avoid this.