Don't forget about AWA/US. This would give three majors on the US playing field.

As others have pointed out UA and CO are very different airlines (equipment, engines, product offerings, etc.) I could see a strong alliance between the two but a total integration promises to be messy. Yet, if an alliance were founded, it certainly would be interesting! Both AA/NW and CO/UA would have access to LHR (as mentioned above), and NRT 5th freedom rights. Exclusive to the CO/UA side there would be hubs in EWR/IAD, CLE/ORD, IAH, SFO, DEN; SYD flights along with other Pacific routes.

Plus, it sure would be nice if a PC membership got you into the RCC!