As far as I know, Minelab has not made any "official announcement" about the availability of future coils. However, there are four different coils shown on page 69 of the X-Terra operator's manual. The drawings appear to represent the currently available 9-inch concentric coil, a similarly sized DD coil, a larger DD coil and a smaller coil.
Based on the pictures in the manual, and the reputation of Minelab, I believe that additional coils, both smaller and larger, will soon be available for the X-Terra. I have no idea which frequency will be available for which size. But, having used both the 7.5 and the 18.75 in the nine-inch concentric, it won't make much difference for the sites I hunt. Both are extremely sensitive. I have found, however, that I must lower the sensitivity setting of the X-50 a couple levels when using the higher frequency coil. JMO HH Randy