I think it would be even better if there is an option to select/change the currency after registration or simply remove the currency selection on registration and let player select the currency under their account setting. I do not think adding such option is difficult for an experienced developer of a gambling site. Also it would be nice if the fast registration option is added to the main site under the registration tab so users who do not want to have complicated registration process can change it to fast registration easily.
Well, I don't see it as a big limit considering that we're in cryptospace and everyone is going to use crypto for deposit, so currency is one of that thing that is no sense. What you purpose will not solve this problem because if you choice your currency from account settings it will be always the definitive one. About fast registration I'm in agree with you

I don't really agree with that. The advantages of depositing and withdrawing in cryptocurrency are undeniable. However, regardless of what I deposit in cryptocurrency, I still find it much more convenient to have my balance and bets displayed in a currency I know or the currency I deposited in, rather than some unknown fiat currency. For example, US dollars and euros are by far the most used currencies in the world so the casino should set one of those two as its default when registering an account. Also, the casino should allow changing the base currency in user settings in case someone makes a mistake during registration.