It would be also cool to make a service which will allow you to bet money on your team before play in official DOTA 2 or CS. But I do not know how to realize it, and not sure if it's legal with a steam company.
there are a lot of places that you can gamble on your favorite teams in Dota2 or CS:GO games but none of them are affiliated with Steam or Valve. they are all third parties. there are casinos that work with bitcoin even, but there are more working with fiat and even more working with in game items (like the cosmetic items you use in Dota2 like the AntiMage weapon or Pudge's hook).
there is also GosuGamers which is one of the biggest gaming related website that I remember it had something called "branches" in dota2 page that you could gamble them (they had no value) and then they added item betting.
as I said I don't know how legal any of these are, but they are happening.
Maybe you are worried if it would be illegal since you are making a new version or your own version of betting site? well there are so many sites out there, just like what @Pursuer said that are not affiliated or related to Steam or Valve. Most of those sites make their own version of E-sport betting site and adding some unique styles to prevent plagiarism or copyright issues. Some sports book offers different while the other is different, and i think that is not prohibited because its beyond the reach of Steam or Valve, and its also your own site so you have the choice to make a difference.