Lubicon, the last US airline I'd see being interested in coming to YEG is Delta or Skywest! In fact, Delta does not have any plans to return to YEG. The only new US airlines may well be AA for the YEG-ORD route and maybe UA for YEG-DEN. YEG-DFW? I agree that probably might not work out well.
I have heard in the past that United had been taking an interest in serving YEG. That might happen when the BAe 146s go within a few years (they're on Milty's hit list of a/c to get rid of, along with the F28s and 732s) If that does happen, either UA may use 737-500s or A319s, or AC may take over with A319s. Perhaps even both if the demand warrants it.
The number of US destinations(5) nonstop from YEG are pretty much to the limit now. There may be one or even two more to be added, but that may not happen for a long time.
Expect the F28s to be gone within a couple of years from the YEG-YVR route - only reason they're still on that route is that they add the capacity to the one afternoon 737-200 flight daily, probably because AC can't spare an A319!
The 146s will stick around a bit longer, but will go eventually. The 737, like the BAe 146s, will stick around a bit longer probably. I'd say by as early as 2005, A319s and CRJs may take over on that route. Remember that AC's regionals will get the CRJs within a few years, as the new pilots' contract states. Since CRJ-200s are smaller a/c than the 146, hourly frequency on the YEG-YVR route may work. CRJs would also be very useful in replacing F28s on YEG-YYC.
I do hope that the YEG-LHR service will last - as Scott Clements, head of Edmonton Airports, has indeed said that it is up to the people of Edmonton to keep this route flying. One of the main reasons AC pulled out of YEG a few years ago was the competition between AC and CP, which likely meant that load factors were too low for either airline. AC couldn't even keep flying 762s on that route! It's also probably a major reason apart from low demand why AC couldn't hold on to those routes that Caribb mentioned earlier. Now with one national airline, I hope AC can keep flying the YEG-LHR route longer.