I don't believe that adding more coin options will bring you more players as there's a lot of gambling site particularly web 3.0 where you can even place bet using some altcoins and some of them were not as successful as the others, if you are already established crypto I think the best way you can add to keep your players loyal is by giving them reward or promotion, who wouldn't want that isn't? Also, from time to time personal interaction with them and probably small gifts would build a relationship, not because you let them play in your site then you would ignore them sometimes building a relationship with your players would give you more profits.