^ I think you need to hire a good advisor and knowledgeable when it comes to planning to build a gambling platform.
Most popular as of now are dice, sports betting, crash game, poker or any cards related games. If you compete for those popular once and already build reputations, I think you need to have something unique. Promotions and faucets or giveaways are the best marketing strategies in gambling industry. Nevertheless, good luck with your journey to building a good gambling platform.