The general answer would be:
UA's widebody program is pretty well committed to the 787/A350 for many years to come. They are *expected* to take deliveries beginning in 2016-ish. And once fully implimented, those airframes should last at minimum 10-15years. SO UA's next widebody order probably will not even be looked at untill 2025 at the earliest.

The "outside chance" answer would be:
Almost zero chance they would aquire the 380. However, I would have said the same thing about the 319/320's..... The only way I see UA getting them is (much like the 319/320's) for Airbus to offer the sun, moon, and stars for UA to "take" them. Also, long term delivery delays in the A350 program could also net UA some A380's.

As far as the 747.... very much in the same spirit as the 380 answer: Not going to happen. Short of an extremely favorable deal to "take" them.