Let's make a Dota 2 tourment, where the event leaders make teams and the winning team gets a reward or something!
I don't think a MOBAs are a very good starting point. A lot of people don't play them, and those that do usually fight over whether LoL or Dota is better. I suggest something simpler, easier to learn and quicker to download. I'd suggest Infinity Wars or Hearthstone, mostly 'cause I love TCG games.
Either way, there's only a small to medium group on this forum that actually plays Dota, so no. But there's no spectate mode in those TGC. I do play Heartstone myself, but i'm not very good at it.
Me, Downcorp and UtD have/had looked at doing this but it mostly fell on its head since it requires ALOT of preplanning and organisation. We'll probably do it eventually, but not soon because its the holiday season.