I would say the best method to grow your Bitcoin would be using your BTC to start a gambling site, since 50% of Bitcoins economy is gambling...it would definitely profit you.
But you also need a large amount of btc's to be invested as no player will play in a gambling site if the site has less bankroll.
You also need a team here includes coders, developers, support team etc. which are also part of your financial expenses regularly together with promoting, advertising, hostname etc.
There's a hidden expenses to so watch out.
There are also traitors to watch out, just like what happened to magical dice recently. They were betrayed and ripped off by their own developer and managed to cheat the system which is the cause of closing of the said site. Choosing a trustworthy team is a must. So for me starting a gambling site not a good idea
Coutnerparty risk for gambling businesses is very very high due to the nature of it