...luckily, I have an alternative.
Don't like it. Don't see a point for it in the lineup at Buick. It's a waste of resources.
Take the normal Regal (added the little side flourish, because the production model is lacking):
then stretch it:
Charge another $5k for the stretch, and bada bing bada boom you just made the LaCrosse functionally obsolete.
A comparison if you can't tell the difference in length:
(apparently the Chinese like gold)
The requisite black:
And the absolute top of the line car as a stretched Regal with a transversely-mounted 400/400 LGW for a GS in... black:
I would see this car topping out as a stretched GS, and with a DIFFERENT sort of Avenir package as more than just an appearance package but rather access to additional colors (possibly including an upcharge for bespoke) inside and out (plus better materials, etc) for ~$65k.
Pick a strategy with it: available like every other model, or maybe limit it/try to make it something special. I've always liked the Marauder, and could see this filling a similar kind of role at Buick. Very low volume.
Give it a name or some neat detailing like the Demon to give people a REASON to buy it. Make people want it because YOU want it. I don't know Buick well enough to figure out what that might be, but this would be a pretty good start.
Also: you would get that LGW in the standard wheelbase, as a GS. There would be no LGX.