ATC has been discussed a lot, and one way that might be worth a thought instead of hard-implementing „pure US ATC“ or „pure ICAO ATC“ or whatever more local variant you could think of:
Since the speech is synthesized anyway, I could think of the supported phrases sit in a text file, with a list of possible placeholders for stuff like aircraft registration, aircraft type, flight number, flight data, met data etc.
As an example, a takeoff clearance like this:
„<ac_reg>, wind <wind_direction> degrees <wind_velocity> knots, runway <cleared_runway>, cleared for takeoff.“
As many things in the new sim seem to be adjustable in text files, this doesn’t feel totally off.
As a (very long term…) extension to this, I could think of regionalized files to accommodate different phraseology in different areas of the globe.
Also I’d love to have different variants of single phrases. Now before anyone jumps at me with „realism, realism! There’s just one way to do it right!“ stuff, I know, I know. But the reality you might claim is (in reality) a mix of phrases, and not adhering to whatever standard 100% of the time.
Also there are things you might say in more plain language, as there is not a given phrase for everything.
Examples for variants:
„<ac_reg>, <turn_direction> heading <cleared_hdg>.“
„<ac_reg>, turn <turn_direction> heading <cleared_hdg>.“
„<ac_reg>, fly heading <cleared_hdg>.“
Again, I’m not saying any of these are per standard XY, I just want to point out that the „real“ experience is a rich one, made up of variations „around“ a particular standard.
So long story short, I vote for customizable phrases in plain text files.
There are miracles happening at last. 
Thanks to