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.
Adding new coins is more often a need for casinos during the bull run, this is because as we know during that time new coins which no one has heard about can grow very rapidly and the holders of those coins may like to begin to use them for something other than just keeping them in their wallets.
That is when we see a demand for casinos to adopt those coins, and doing so as fast as possible can in fact give a big advantage to a casino over another, as they can gain a bunch of new clients by just adopting this new coin, clients which they can keep for a very long time with the use of bonuses and free bets.
In these moments, as the market is, it is good to add other currencies, why?there are many that are in projects, that have a good level to increase, also if one of those projects manages to enter a casino it is likely that the casino as the Owners of the currency can do incredible things, bring an entire Community of a project to a casino and apart from the traffic and movenments that will be given to the coin will be incredible, without thinking that the bitcoin when this currency rises,but even so it is Cheaper to play with the currency to avoid the high fees.