The only recycled denominational letters possible that remain are A, B, C, F, G, and U. I have a hunch that AUA (which once appeared on the $2 bills of 1986) will follow APZ on the $5s. After ARZ they may backtrack to the earlier denominational letters with AA*, AB*, etc., or they may go to BA*, BB*, etc. There's usually around a 20 year gap between the release of the prefixes on the $1 and $2 bills and on a current denomination, so it's highly unlikely we will see AA*, AB*, etc. since those prefixes were first seen nearly 30 years ago, but anything is possible.
As for the $10 bill, seeing that the $10 has been falling into decline for many years now, I wouldn't be one bit surprised if they choose not to recycle a retired denominational letter and go back to the earliest denominational letter for that specific denomination - which is "D" - and go to CDA after BTZ. If we see CDA, that may be a sign that the end may be nearing for our beloved purple banknote, though it may not be formally announced until thereafter.
