I'll tell ya what Buick needs... let me preface this by saying that all of these vehicles should expound on the Velite and Enclave styling themes. It's just my thoughts, but Buick doesn't need to invent the wheel. Use the Zeta (or whatever the hell afordable RWD platform is coming to NA)... All of these cars should handle well (without necessarily being very sporty; leave that to Caddy). They should be quiet, and powerful. Tasteful and refined. Any reference I use to the V6 is the 270HP HFV6 used in the Enclave; the V8 should be the N* rated @350HP. Future Caddy's should use a version of the V6 at around 300HP, and a V8 at around 400HP now (to remain competitive with the Germans and Lexus/Infiniti). That means the Buick will be powerful, but can be offered at a value compared to the competition, while also not stepping on Caddy's toes.
The next Lacrosse should be smaller, RWD. Use the 3.6 from the Lambda. Make it as plush and well appointed as possible. It should be full loaded, with very few options (like Navi).
The nex Lucerne should be a larger car on the same platform; Basically, do the same as with the Lacrosse on the next level up; sell it for $37K(V6), $39.5 (V8), and have just a few options again, with fully optioned version selling at around $40K for the V6; $44 for the V8.
Flagship @ $48K. Have niceities like Navi standard, and leave a few options like larger wheels. It should be the size of the S or 7, and be quieter than the LS (with just as many appointments.
The next gen Theta SUV. It should use the 3.6L V6 found in the rest of the range. It should be priced at $28-29K (the Caddy Theta should be $31-32K to start).
Enclave, ofcourse. V6 starting at $35-36K; V8 a few grand more.
Velite - A hard top convertible using the RWD Lucerne's platform; V8 only, quiet and powerful; more of a SC competitor than a SL/Xlr compeitior. If the powers that be think that another lux hard top convertible from GM is overkill, make it a coupe version of the Lucerne; a blvd cruiser that would be seen as an alternative to the 6-Series coupe.
As far as Caddy, they're pricing structure would be much more similar to MB's etc., offering more options and a sportier performance. Also, it'd have a fuller lineup, with the Escalade and V-Series... so, there's room for two lux makes at GM, IMO.