Part of the issue with promotional units was their nature as "limited-run, buy-now-or-forever-miss-out". I suggest changing them to a "limited availability for in-game resources, permanently available for nanos" model. My proposal would work as follows:
- Upon (re)introducing a new unit, make the sale price cost in-game resources: Gold, Steel, Oil, Gears/Bars/Teeth/Skulls, etc. as appropriate.
- After one or two weeks, change the price to nanopods, with prices ranging from 50-90, depending on the unit.
This could also require a special training facility (unlocked after completing one of two objectives:
- Clearing the mission "Warlord Gantas"
- Defeating the Ancient Construct for the first time
This would work well for making the units available to players who otherwise may have completely missed them, and might increase the amount of income via nanopods without using extreme emotional manipulation via FOMO to accomplish it.