They may or may not use "X" but they need to differentiate the base level chips from the mid level and high end.I tend to doubt that Apple will continue the 'X' designation with the M series.
Something like this. Exact naming would be a marketing decision.
features | base configuration | more RAM, more cores, more GPU | More more more of everything | |
machines | Air/Mini/iMac | MBP/large iMac | Mac Pro | |
initial series cores based on A14 | M1 | M1x | 2021 | |
faster core design based on A15 | M2 | M2x | M2z | 2022 |