Lots of room here for ideas, figured I'd start it off with a few that I think would have been really cool. Maybe wouldn't have sold a ton (probably why they weren't produced), but just as a "what if", it's fun to consider.
*Chrysler 300C Hellcat - yeah, they made some SRT 300's, but dropped them (in the US, anyway) after a few years. Imagine if they had shoehorned the Hellcat Hemi into the 300C, but kept it (at least partially) subtle, understated, refined-less "baller dubs" and more "Q ship/gentlemens' express". Yeah, I know several people have done their own swaps, and that's cool. Does anyone know if it's possble to combine the Hellcat drivetrain (engine/trans) with the AWD system some of the 300's have? That would put the power to the ground, provided the transfer case and halfshafts don't twist into pretzels.
*Cadillac ATS-V(8). Great chassis, nice bod, but a v6. Yes, it was turbocharged, intercooled, and direct injected....but it wasn't a V8. Drop the LS9 in one and say hallelujah.
Pontiac G8/Chevy Caprice/SS with a CTS-V drivetrain. Easy peasy. I daily drive a G8 GT, and it's fun, but close to double the horsepower would be even more fun.
Dodge/Ram/FCA/Stallantis/whatever full size SUV - The Durango was OK, but why didn't they produce a full-size to play with the Tahoe/Suburban/Expedition class of SUV's? They had the platform (Ram trucks), the engines (Ram trucks, SRT Hemis, etc) but I guess decided the Durango was as far as they wanted to go? Hmmm.
Chevy Blazer based on a shortened Tahoe chassis. Get rid of the rear doors, lose a foot or so of wheelbase, and voila - Blazer.
Allow me my inebriated musings, and share yours.