Many people seem to think bigger = better, but that's just not true. There's no point adding 10-15k seats to BMO field (beyond the approx 40k needed for the World Cup) if it will ruin the atmosphere for the next two decades of TFC games. No one's going to say "Oh, the World Cup would've been great, but those three Group Stage games in Toronto were awful because there were 10k less people in the stands than if they had an NFL stadium (even though I only saw the stands on TV for a brief moment) and it ruined the whole competition and my image of Canada..."
If it's a good game then it won't matter whether there's 40k or 60k people. Same if it's a bad game. And frankly, it's not like we're auditioning to hold another World Cup any time soon - it won't be coming back to Canada for at best 20-30 years.
An advantage of BMO field is the huge parking lot area/Ex grounds around the stadium. I hope they move parking off-site and set up huge screens for viewing parties. It would be an epic view and create a great atmosphere for the fans, both in and outside the stadium.
Also, I may be wrong but I don't think the SkyDome is even an option for the World Cup. The arrangement of the stands is just too awkward for a soccer field. NFL stadiums would work because the field is similarly shaped (and therefore sightlines are the same), but MLB stadiums are different (so far as I remember, it was "okay" for the Argos because they didn't open up the upper bowl). While it could theoretically work, I'd imagine FIFA wouldn't allow such a hodgepodge arrangement of stands/sightlines in a World Cup match (not to mention I'd imagine FIFA has pretty hefty requirements regarding the grass, which BMO Field can make work while SkyDome couldn't due to the baseball base-running dirt parts (there's gotta be a proper term for that, but I can't think of it...)).