I'm really having a hard time picturing where a convention centre could go in the Metrotown area...
Convention centres are extremely pedestrian-unfriendly and should be kept to the "edge" of the town centre if possible. In most cities, the area around a convention centre is pretty dead - they usually create a bit of a void of activity. Our convention centre downtown is an oddball in that it hasn't created the same problems that others do.
For one, it fits in well with its surroundings. Most of the building is actually below street level due to the escarpment, so you don't have this overbearing structure like you do with BC Place, for example. It's also at the edge of downtown (literally), so it's not blocking off any streets, or any "flow" of people. And of course, it's surrounded by the Seawall, which happens to be one of the city's most popular areas for bikes and pedestrians.
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Anyway, I can't see it being south of Beresford, which is heavily residential. Maybe consolidate the little Maywood school with the adjacent low-rises?
Otherwise, I can only really see it being along Kingsway or Willingdon. There is officedweller's idea along Willingdon, which, if it includes the Esso block, could be a nice location. Same goes for the Old Orchard site, which could also be combined with the Earl's site. There's also the retail/office complex on the SE corner of Kingsway & Nelson, but that's getting pretty far from the SkyTrain station.
The Sears property is also quite far, but it's pretty central along Kingsway, so it could be a good choice. If Burnaby is looking at building this thing in the near-ish term, then this might well be the site, since it appears to be the only one that's really in play at the moment.
Here's a quick map of the locations I mentioned. BLUE = Old Orchard/Earl's; ORANGE = OD's site; YELLOW = Maywood School; RED = Sears; GREEN = SE Kingsway & Nelson.