It would be really cool to have the 2019 or 2021 tournament in Winnipeg, although as mentioned the second arena would really pose a problem.
Brandon, Manitoba (Keystone Centre, 5,102 capacity): A 2 hour drive from Winnipeg, probably the most logical choice if you want to keep the second arena in the province.
Selkirk, Manitoba (Selkirk Rec Centre, 2,751 capacity (not sure if this includes standing room)): About a 45 minute drive from downtown Winnipeg as someone else mentioned, and has hosted/ co-hosted other major tournaments such as the 2002 World Under 17, and the 2007 Women's World Championship. However, it is way to small for a tournament of this size. If Selkirk had a 5,000 seat arena and a WHL team it would be plausable.
Regina, Saskatchewan (Brandt Centre, 6,200 capacity): Larger than Brandon, but too far away from Winnipeg to realistically work.
Grand Forks, North Dakota (Ralph Engelstad Arena, 11,643 capacity): A little over a 2 hour drive from Winnipeg (depending on the time at the border), it is the largest arena nearby even though it is in the USA. The idea intrigues me about a split Canada/USA hosted tournament and arena size would not be an issue, but I still couldn't see it happening in the near future.
Winnipeg, Manitoba (MTS Iceplex, 1,350 capacity): The Jets practice facility would be too small to host this type of tournament.
Portage la Praire/ Winkler/ Dauphin, Manitoba: Too small arenas