The top brass of Flutter (FLTR:LN) got some bad news over their Christmas break: DraftKings, Flutter’s biggest rival in the US, is planning to go public in 2020. DraftKings will merge with an existing listed business, putting it on the public markets without the hassle of an IPO. Once on the public markets, DraftKings will find it easier to raise money. And the battle for the brand-new US sports betting industry will require plenty of that.  The legalisation of sports betting in US states is a giant, never-to-be-repeated opportunity for sports betting companies like Flutter. By 2028, the US market…