i think sportsbet.io promos is unfair, because always the winners is who have most of btc, and not basically about skills !!! maybe rules like minimum 10 mbtc and highest odd will win iS better.
It is quite fair in my point of view, with more bitcoin on the stake you are risking more so you should habe much better chance in winning than those who risk lower than 10 mbtc. It might not seems fair for some people however your system that you proposed should be much unfair for those who risk more