Out of curiosity, does XPZ 'increase' in difficulty like Long War? Where every month, other factions get 'research' and 'upgrades'? Looking through the conf files, it appears that certain UFOs spawn more as the years go by.
OpenXcom supports alien tech levels, but XPZ is using only a single one... so no, there is no progression in this particular way.
You can fight against blaster launchers in January 2601 (see FireWaterGasoline's LP)... just like you can play against shovels in December 2601 (see my LP).
However Dioxine is removing the early game madness (e.g. hideout defenses)... by not giving you means of shooting down ships... so you'll feel a little curve in newer XPZ versions.
Still, the rest is mostly just RNG.