I have entered some of Unearthed Arcana feats and the ones that are based on skills have the following ability, "You gain proficiency in the X skill. If you are already proficient in the skill, you add double your proficiency bonus to checks you make with it." Now I know this isn't technically "Expertise" but let's call a spade a spade, it's basically the same thing. 

So here is the issue. If my character has chosen Intimidation to be proficient in as a race/class feature. There is no way to homebrew the above caveat, at least not to my knowledge. If I add a modifier to that feat that grants proficiency or expertise in Intimidation, then once that feat is chosen, it then resets the chosen proficiency in Intimidation as the race/class selection. So essentially, if a player forgets that proficiency is 'supposed' to be Intimidation, then they could basically gain an additional proficiency by selecting another one.

Now, in the meantime, I can just remove the proficiency/expertise modifier on the feat, keep Intimidation as the selected race/class skill selection, then "Customize" intimidation on the character sheet, granting the expertise. HOWEVER, this will then turn the Intimidation dot Blue and not black along with the rest of the skills, which irks my OCD.

BASICALLY! I would really like to know if there is a way to grant the "You gain proficiency in the X skill. If you are already proficient in the skill, you add double your proficiency bonus to checks you make with it." to homebrew modifiers, without replacing the selected proficiency, or having it be customized and turned blue.

There is no way to do this. The way I've done it for some homebrew I've needed is I've made 2 feats. One that you choose if you have (skill) already, and one you choose if you don't. It's not the best, but it does make the feat work as well as possible.

I imagine we won't get a modifier like this until WOTC makes an official feature that emulates it-- Just a guess.

But if I give the feat expertise with Intimidation and the character (human variant) chooses intimidation as their skill proficiency, then the feat removes intimidation from the skill selection, leaving that space open with a red exclamation mark signalling that theres' an option available. I would choose a random skill that the character wasn't used, but the character is a bard, and therefore gets half proficiency to everything and if I turn that proficiency to a half proficiency then that half circle is blue too and there's still the issue :/

Well this is just the weirdest thing ever....it has been resolved. Intimidation now has black expertise and the chosen skill remains intimidation.....very interesting.

Thanks you guys!