The Sigma thing is the culprit here - it cannot be stretched too much, so CTS and STS had to be similar in size. They ended up CTS being too big for the 3er and STS being to small for the 7er. Traditionally, the Seville was regarded as 7er/S-Klasse competitor in Europe (even if it fared miserably), so it's just a matter of growing a bit in size and a lot in quality and design
The "BTS" would require a new platform, probably, or an extensively reengineered Sigma. And a serious name, of course
"The Europeans" embraced the A-Klasse and A3 to some extent, but the sales of the 1er are lagging behind BMW's expectations (no wonder, as the 1er's main competitor is 3er Compact
). There is a market for premium compact cars in Europe, but I believe that "midmarket" premium brands are better suited to compete here - I believe a Saab 9-2 could take care of the A3, 147, C30 and the like - no need for Cadillac to go that low. The same applies to "BRX".
Trying to sell a model priced like the Continental and justifying the costs of a brand-new state-of-the-art V12 could be a hard task for Cadillac, until all other problems are rectified. I think successfully fighting the S-Klasse should be on Cadillac's agenda for now. When they come to the moment when they will be able to compete with Bentley, petroleum engines might be history already.
Do also remember that it's not a football game, when you need to have a pre-set number of players. The fact that competition has a particular model portfolio does not mean that you have to have such, it all depends on your brand image and focus. BMW has so many models becuase they have one and a half brands, and Mercedes' prolific model portfolio resulted from Stuttgart boys' big egos - not long ago we were reading of the R-Klasses misfortunes.