Well, if I may lend my armchair genius, this is what I'd do:
A new twin-turbo Cadillac V-8, which would support both more volume-oriented (Vsport level) and higher end (V-Series level) variations, and therefore output could range wildly. The lower output versions would be shared with the Corvette Stingray, although of course the Vette would have its own exhaust tuning, etc. The Z06/CTS-V sharing of a high-output version would continue. Although the CTS-V wouldn't really need much more sheer horsepower and torque, the torque band of the twin-turbo V-8 would probably provide superior performance nonetheless (The CTS-V already has its hands full during acceleration with, say, an RS7, even at high speeds, in spite of its considerable power advantage, so added performance isn't pointless). This great engine would also enhance the Corvette's performance. Cadillac really wants to market themselves as a performance luxury brand. While I don't share that specific vision, this presents an obvious opportunity: all the easier to market the Cadillacs that share "a Corvette engine".
As for other things the CTS-V could use, an AWD option of some kind and carbon ceramic braking would be worthwhile. Oh, and keep up the limited editions.