Well, I think the TFC point is up in the air. Sure, you can hold one off games between huge European Clubs and have a great atmosphere (having been to EPL games, TFC's current crowd provides as good an atmosphere already), but will the game grow to allow for a season's worth of demand at 55k? I don't think it's projectable at this time anyways so we'd just be debating the popularity of soccer in Canada which hasn't really gone far beyond youth participation.
UFC was a huge event but again, you can't hold those all the time. The fact it was the first one with a massive card and pent up demand really helped. That's not to say the next one won't sell out and the next one after that and son on, but scarcity drives demand.
As for the idea of 4 or 5 rinks on the ground level, it's an interesting idea but I wonder how difficult it would be to keep a building of that size cold enough to maintain those rinks? I know they did it in Tampa when the Lightning joined the league and played out of the Trop where the Rays play, but I don't know the specifics of how easy or difficult it was to maintain the rink there. I guess my point is, any renovation to skydome, no matter its new use will be a pretty big undertaking.