One of the warlock's Eldritch Invocations, Beguilling Influence, bugs me. The full description is:
You gain proficiency in the Deception and Persuasion skills.
This is fine for a warlock who doesn't already have proficiency with those two skills, but it's a bit weak for someone who already has one of them, and entirely useless for someone who already has both of them.
I was wondering whether it might help to get some more mileage out of this invocation if I were to change it to this:
You gain proficiency in the Deception and Persuasion skills. If you are already proficient in either of those skills, you instead gain expertise with that skill, which means your proficiency bonus is doubled for any ability check you make with it, unless it is already benefiting from a feature, such as Expertise, that doubles your proficiency bonus.
The intention is that it wouldn't be any different for a warlock who wasn't already proficient, but for a character who was already quite deceptive and/or persuasive, this invocation would enhance their abilities even further. This, to my mind, fits with what I believe is the implied flavour of this invocation; that you are being supernaturally enhanced by your patron to be more deceptive/persuasive.
I added the clause about it not stacking with Expertise (wording borrowed from the Prodigy feat) so that it couldn't be exploited by a warlock/bard or some other combination.
My question is, does this seem like a good, clean change that would add to the game, or are there issues that would make this problematic, either 1) in terms of balance or 2) going against the design of what invocations are for, or 3) that would make it far less impactful than I'm expecting?