Will there be the next regulation after regulation Z?

There is no official information explicitly confirming what happens after the "Z" regulation mark.

However, based on the established alphabetical sequence (D, E, F, G, H, I...) and the history of the Japanese TCG which began with "A", the system is widely expected to loop back to "A".

And also it will take almost 9 years to reach the "Z" regulation mark from the start of the "D" era, based on the current release rate of 3 new letters per year... so it's something they have time rethink.